feat: split a transaction across multiple categories

Один чек часто смешивает несколько категорий трат (гипермаркет: часть — продукты,
часть — бытовая химия). Раньше такую транзакцию приходилось огрублять до одной
категории или дробить вручную на несколько записей. Теперь у транзакции есть
второй режим: 2+ доли (TransactionSplit) с суммами по разным категориям, которые
обязаны в точности складываться в сумму транзакции.

- Новая модель TransactionSplit + миграция.
- split_rule_violation() проверяет инварианты на создании и редактировании.
- category_rollup.py — общий helper, которым теперь пользуются Dashboard, отчёт
  по категориям и бюджеты, учитывая доли сплитов наравне с обычными транзакциями.
- Фильтр по категории в списке транзакций находит и сплит-транзакции.
- Сплиты попадают в полный бэкап/восстановление БД.
- 13 новых backend-тестов.
- Фронтенд: переключатель «Разбить на несколько категорий» в форме транзакции
  с повторяемыми строками и индикатором остатка суммы; строка транзакции в
  списке показывает все категории сплита.

backend/alembic/env.py

@@ -22,6 +22,7 @@ from app.models import (  # noqa: F401 — registers metadata
     RecurringTransaction,
     Tag,
     Transaction,
+    TransactionSplit,
 )
 
 config = context.config

backend/alembic/versions/7b535ef9e189_add_transaction_splits.py

@@ -0,0 +1,36 @@
+"""add transaction splits
+
+Revision ID: 7b535ef9e189
+Revises: 3e3503b61b20
+Create Date: 2026-08-30 15:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '7b535ef9e189'
+down_revision: Union[str, None] = '3e3503b61b20'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'transaction_splits',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('transaction_id', sa.Integer(), nullable=False),
+        sa.Column('category_id', sa.Integer(), nullable=True),
+        sa.Column('amount', sa.Numeric(precision=14, scale=2), nullable=False),
+        sa.Column('note', sa.String(length=200), nullable=True),
+        sa.ForeignKeyConstraint(['transaction_id'], ['transactions.id'], ondelete='CASCADE'),
+        sa.ForeignKeyConstraint(['category_id'], ['categories.id'], ondelete='SET NULL'),
+        sa.PrimaryKeyConstraint('id'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('transaction_splits')

backend/app/api/routes/transactions.py

@@ -1,4 +1,5 @@
 from datetime import date as date_
+from decimal import Decimal
 from typing import Literal
 
 from fastapi import APIRouter, Depends, HTTPException, Query
@@ -10,20 +11,27 @@ from app.api.deps import get_session
 from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
 from app.models.tag import Tag
-from app.models.transaction import Transaction
+from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.transaction import (
     TransactionBulkCreate,
     TransactionBulkCreateResult,
     TransactionCreate,
     TransactionPage,
     TransactionRead,
+    TransactionSplitInput,
     TransactionUpdate,
+    split_rule_violation,
     transfer_rule_violation,
 )
 
 router = APIRouter(prefix="/transactions", tags=["transactions"])
 
-_EAGER = (selectinload(Transaction.account), selectinload(Transaction.category), selectinload(Transaction.tags))
+_EAGER = (
+    selectinload(Transaction.account),
+    selectinload(Transaction.category),
+    selectinload(Transaction.tags),
+    selectinload(Transaction.splits).selectinload(TransactionSplit.category),
+)
 
 
 async def _resolve_tags(session: AsyncSession, tag_ids: list[int]) -> list[Tag]:
@@ -62,6 +70,14 @@ async def _ensure_category_matches_type(
         )
 
 
+async def _build_splits(
+    session: AsyncSession, splits: list[TransactionSplitInput], transaction_type: TransactionType
+) -> list[TransactionSplit]:
+    for split in splits:
+        await _ensure_category_matches_type(session, split.category_id, transaction_type)
+    return [TransactionSplit(category_id=s.category_id, amount=s.amount, note=s.note) for s in splits]
+
+
 @router.get("", response_model=TransactionPage)
 async def list_transactions(
     year: int | None = Query(default=None, ge=2000, le=2100),
@@ -97,8 +113,15 @@ async def list_transactions(
         stmt = stmt.where(Transaction.account_id == account_id)
         count_stmt = count_stmt.where(Transaction.account_id == account_id)
     if category_id is not None:
-        stmt = stmt.where(Transaction.category_id == category_id)
-        count_stmt = count_stmt.where(Transaction.category_id == category_id)
+        # A split transaction has category_id=NULL on the row itself — the
+        # category lives on its split lines instead, so filtering by exact
+        # column match alone would silently drop it from a category filter
+        # it genuinely belongs to.
+        category_filter = or_(
+            Transaction.category_id == category_id, Transaction.splits.any(TransactionSplit.category_id == category_id)
+        )
+        stmt = stmt.where(category_filter)
+        count_stmt = count_stmt.where(category_filter)
     if tag_id is not None:
         stmt = stmt.where(Transaction.tags.any(Tag.id == tag_id))
         count_stmt = count_stmt.where(Transaction.tags.any(Tag.id == tag_id))
@@ -151,9 +174,11 @@ async def list_transaction_years(session: AsyncSession = Depends(get_session)) -
 @router.post("", response_model=TransactionRead, status_code=201)
 async def create_transaction(payload: TransactionCreate, session: AsyncSession = Depends(get_session)) -> Transaction:
     await _ensure_category_matches_type(session, payload.category_id, payload.type)
-    fields = payload.model_dump(exclude={"tag_ids"})
+    fields = payload.model_dump(exclude={"tag_ids", "splits"})
     transaction = Transaction(**fields)
     transaction.tags = await _resolve_tags(session, payload.tag_ids)
+    if payload.splits:
+        transaction.splits = await _build_splits(session, payload.splits, payload.type)
     session.add(transaction)
     await session.commit()
     refreshed = await session.execute(
@@ -171,11 +196,17 @@ async def bulk_create_transactions(
     instead of leaving a half-imported statement behind."""
     for item in payload.items:
         await _ensure_category_matches_type(session, item.category_id, item.type)
+        for split in item.splits or []:
+            await _ensure_category_matches_type(session, split.category_id, item.type)
 
     transactions = []
     for item in payload.items:
-        transaction = Transaction(**item.model_dump(exclude={"tag_ids"}))
+        transaction = Transaction(**item.model_dump(exclude={"tag_ids", "splits"}))
         transaction.tags = await _resolve_tags(session, item.tag_ids)
+        if item.splits:
+            transaction.splits = [
+                TransactionSplit(category_id=s.category_id, amount=s.amount, note=s.note) for s in item.splits
+            ]
         transactions.append(transaction)
 
     session.add_all(transactions)
@@ -187,13 +218,16 @@ async def bulk_create_transactions(
 async def update_transaction(
     transaction_id: int, payload: TransactionUpdate, session: AsyncSession = Depends(get_session)
 ) -> Transaction:
-    # Eager-loads tags — assigning transaction.tags below would otherwise lazy
-    # -load the current collection first to diff against, which async
-    # SQLAlchemy can't do outside an explicit await (MissingGreenlet).
-    transaction = await session.get(Transaction, transaction_id, options=[selectinload(Transaction.tags)])
+    # Eager-loads tags and splits — assigning transaction.tags/splits below
+    # would otherwise lazy-load the current collection first to diff
+    # against, which async SQLAlchemy can't do outside an explicit await
+    # (MissingGreenlet).
+    transaction = await session.get(
+        Transaction, transaction_id, options=[selectinload(Transaction.tags), selectinload(Transaction.splits)]
+    )
     if transaction is None:
         raise HTTPException(status_code=404, detail="Transaction not found")
-    updates = payload.model_dump(exclude_unset=True, exclude={"tag_ids"})
+    updates = payload.model_dump(exclude_unset=True, exclude={"tag_ids", "splits"})
     # Checks run against the row as it would look after the patch, not just
     # the fields sent: switching type alone can invalidate fields left
     # untouched.
@@ -201,6 +235,7 @@ async def update_transaction(
     effective_category_id = updates.get("category_id", transaction.category_id)
     effective_account_id = updates.get("account_id", transaction.account_id)
     effective_transfer_account_id = updates.get("transfer_account_id", transaction.transfer_account_id)
+    effective_amount = updates.get("amount", transaction.amount)
     await _ensure_category_matches_type(session, effective_category_id, effective_type)
     violation = transfer_rule_violation(
         type=effective_type,
@@ -210,10 +245,36 @@ async def update_transaction(
     )
     if violation:
         raise HTTPException(status_code=400, detail=violation)
+
+    if payload.splits is not None:
+        split_count = len(payload.splits)
+        split_total = sum((s.amount for s in payload.splits), Decimal("0")) if payload.splits else None
+    else:
+        # Splits weren't touched by this patch — check the existing rows as
+        # they stand. Reading straight off the ORM objects here (not
+        # rebuilding TransactionSplitInput) on purpose: an existing split's
+        # category_id can be None if that category was since deleted, and
+        # the sum check below needs none of that — only tags/category
+        # values that were actually just fetched from the caller could ever
+        # need to be re-validated, and those go through payload.splits above.
+        split_count = len(transaction.splits)
+        split_total = sum((s.amount for s in transaction.splits), Decimal("0")) if transaction.splits else None
+    split_violation = split_rule_violation(
+        type=effective_type,
+        amount=effective_amount,
+        category_id=effective_category_id,
+        split_count=split_count,
+        split_total=split_total,
+    )
+    if split_violation:
+        raise HTTPException(status_code=400, detail=split_violation)
+
     for field, value in updates.items():
         setattr(transaction, field, value)
     if payload.tag_ids is not None:
         transaction.tags = await _resolve_tags(session, payload.tag_ids)
+    if payload.splits is not None:
+        transaction.splits = await _build_splits(session, payload.splits, effective_type)
     await session.commit()
     refreshed = await session.execute(
         select(Transaction).options(*_EAGER).where(Transaction.id == transaction_id)

backend/app/models/__init__.py

@@ -7,7 +7,7 @@ from app.models.goal import Goal, GoalContribution
 from app.models.recurring import RecurringTransaction
 from app.models.settings import AppSettings
 from app.models.tag import Tag
-from app.models.transaction import Transaction
+from app.models.transaction import Transaction, TransactionSplit
 
 __all__ = [
     "Account",
@@ -24,4 +24,5 @@ __all__ = [
     "RecurringTransaction",
     "Tag",
     "Transaction",
+    "TransactionSplit",
 ]

backend/app/models/transaction.py

@@ -35,3 +35,32 @@ class Transaction(Base, TimestampMixin):
     transfer_account: Mapped["Account | None"] = relationship(foreign_keys=[transfer_account_id])
     category: Mapped["Category | None"] = relationship(back_populates="transactions")
     tags: Mapped[list["Tag"]] = relationship(secondary=transaction_tags, back_populates="transactions")
+    splits: Mapped[list["TransactionSplit"]] = relationship(
+        back_populates="transaction", cascade="all, delete-orphan", order_by="TransactionSplit.id"
+    )
+
+
+class TransactionSplit(Base):
+    """One category's slice of a transaction whose amount is divided across
+    several categories (one receipt, several kinds of goods) — an
+    alternative to Transaction.category_id, not an addition to it: a split
+    transaction has category_id=NULL and two or more of these instead, and
+    their amounts must add up to the parent's amount exactly (see
+    schemas/transaction.py's split_rule_violation).
+
+    category_id is nullable + SET NULL, same as Transaction.category_id
+    itself — deleting a category must not break *reading* a split that used
+    to point at it, only creating/editing one requires a live category (see
+    routes/transactions.py, and the same lesson already applied to
+    transfer_account_id)."""
+
+    __tablename__ = "transaction_splits"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    transaction_id: Mapped[int] = mapped_column(ForeignKey("transactions.id", ondelete="CASCADE"), nullable=False)
+    category_id: Mapped[int | None] = mapped_column(ForeignKey("categories.id", ondelete="SET NULL"), nullable=True)
+    amount: Mapped[Numeric] = mapped_column(Numeric(14, 2), nullable=False)
+    note: Mapped[str | None] = mapped_column(String(200), nullable=True)
+
+    transaction: Mapped["Transaction"] = relationship(back_populates="splits")
+    category: Mapped["Category | None"] = relationship()

backend/app/schemas/backup.py

@@ -70,6 +70,16 @@ class TransactionBackup(BaseModel):
     tag_ids: list[int] = Field(default_factory=list)
 
 
+class TransactionSplitBackup(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    transaction_id: int
+    category_id: int | None
+    amount: Decimal
+    note: str | None
+
+
 class AssetBackup(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -202,6 +212,9 @@ class BackupPayload(BaseModel):
     # cleanly under the same format version.
     tags: list[TagBackup] = Field(default_factory=list)
     transactions: list[TransactionBackup]
+    # Defaulted so a backup exported before transaction splitting existed
+    # still imports cleanly under the same format version.
+    transaction_splits: list[TransactionSplitBackup] = Field(default_factory=list)
     assets: list[AssetBackup]
     asset_valuations: list[AssetValuationBackup]
     # Defaulted so a backup exported before crypto holdings existed still

backend/app/schemas/transaction.py

@@ -39,6 +39,37 @@ def transfer_rule_violation(
     return None
 
 
+def split_rule_violation(
+    *,
+    type: TransactionType,
+    amount: Decimal,
+    category_id: int | None,
+    split_count: int,
+    split_total: Decimal | None,
+) -> str | None:
+    """The split invariants in one place, same shape as transfer_rule_violation
+    above — checked on create, and against the row *as it would look after
+    a patch* on update (see routes/transactions.py), so an edit can't leave
+    a transaction whose splits no longer add up to its own amount.
+
+    Takes counts/totals rather than the split objects themselves: the
+    update path's "splits weren't touched by this patch" case has to check
+    the existing ORM rows, whose category_id can be None (the category was
+    since deleted) — nothing here needs a live category to check the sum.
+    """
+    if split_count == 0:
+        return None
+    if type == TransactionType.TRANSFER:
+        return "splits are not valid for transfer transactions"
+    if category_id is not None:
+        return "category_id must be omitted when splitting a transaction across categories"
+    if split_count < 2:
+        return "splitting a transaction needs at least 2 categories"
+    if split_total != amount:
+        return f"split amounts ({split_total}) must add up to the transaction amount ({amount})"
+    return None
+
+
 class TransactionFields(BaseModel):
     """A transaction's shape, without the rules that only make sense while
     writing one. Reading goes through this: rows already in the database can
@@ -83,8 +114,34 @@ class TransactionBase(TransactionFields):
         return self
 
 
+class TransactionSplitInput(BaseModel):
+    category_id: int
+    amount: Decimal = Field(gt=0, max_digits=14, decimal_places=2)
+    note: str | None = Field(default=None, max_length=200)
+
+
 class TransactionCreate(TransactionBase):
     tag_ids: list[int] = Field(default_factory=list)
+    # None/omitted -> a normal single-category transaction, unchanged from
+    # before. 2+ entries -> the amount is divided across categories instead
+    # of using category_id (which must then be omitted — see
+    # split_rule_violation). A single entry isn't accepted: that's just
+    # category_id with extra steps.
+    splits: list[TransactionSplitInput] | None = None
+
+    @model_validator(mode="after")
+    def _validate_splits(self) -> "TransactionCreate":
+        splits = self.splits or []
+        violation = split_rule_violation(
+            type=self.type,
+            amount=self.amount,
+            category_id=self.category_id,
+            split_count=len(splits),
+            split_total=sum((s.amount for s in splits), Decimal("0")) if splits else None,
+        )
+        if violation:
+            raise ValueError(violation)
+        return self
 
 
 class TransactionUpdate(BaseModel):
@@ -99,6 +156,10 @@ class TransactionUpdate(BaseModel):
     date: date_ | None = None
     # Omitted -> tags untouched; sent (even as []) -> replaces the full tag set.
     tag_ids: list[int] | None = None
+    # Omitted -> splits untouched; sent (even as []) -> replaces the full
+    # split set (send [] together with a category_id to turn a split
+    # transaction back into a normal single-category one).
+    splits: list[TransactionSplitInput] | None = None
 
     @field_validator("description")
     @classmethod
@@ -106,6 +167,16 @@ class TransactionUpdate(BaseModel):
         return capitalize_first_letter(value) if value is not None else None
 
 
+class TransactionSplitRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    category_id: int | None
+    category: CategoryRead | None = None
+    amount: Decimal
+    note: str | None
+
+
 class TransactionRead(TransactionFields):
     model_config = ConfigDict(from_attributes=True)
 
@@ -113,6 +184,7 @@ class TransactionRead(TransactionFields):
     account: AccountRead
     category: CategoryRead | None = None
     tags: list[TagRead] = Field(default_factory=list)
+    splits: list[TransactionSplitRead] = Field(default_factory=list)
 
 
 class TransactionPage(BaseModel):

backend/app/services/backup_service.py

@@ -26,7 +26,7 @@ from app.models.goal import Goal, GoalContribution
 from app.models.recurring import RecurringTransaction
 from app.models.settings import AppSettings
 from app.models.tag import Tag
-from app.models.transaction import Transaction
+from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.backup import (
     AccountBackup,
     AppSettingsBackup,
@@ -42,6 +42,7 @@ from app.schemas.backup import (
     RecurringTransactionBackup,
     TagBackup,
     TransactionBackup,
+    TransactionSplitBackup,
 )
 
 BACKUP_FORMAT_VERSION = 1
@@ -52,6 +53,7 @@ async def build_backup(session: AsyncSession) -> BackupPayload:
     categories = (await session.execute(select(Category))).scalars().all()
     tags = (await session.execute(select(Tag))).scalars().all()
     transactions = (await session.execute(select(Transaction).options(selectinload(Transaction.tags)))).scalars().all()
+    transaction_splits = (await session.execute(select(TransactionSplit))).scalars().all()
     assets = (await session.execute(select(Asset))).scalars().all()
     valuations = (await session.execute(select(AssetValuation))).scalars().all()
     crypto_holdings = (await session.execute(select(CryptoHolding))).scalars().all()
@@ -75,6 +77,7 @@ async def build_backup(session: AsyncSession) -> BackupPayload:
             )
             for row in transactions
         ],
+        transaction_splits=[TransactionSplitBackup.model_validate(row) for row in transaction_splits],
         assets=[AssetBackup.model_validate(row) for row in assets],
         asset_valuations=[AssetValuationBackup.model_validate(row) for row in valuations],
         crypto_holdings=[CryptoHoldingBackup.model_validate(row) for row in crypto_holdings],
@@ -110,6 +113,13 @@ def _validate_references(payload: BackupPayload) -> None:
             if tag_id not in tag_ids:
                 raise HTTPException(400, f"Transaction {t.id} references unknown tag_id {tag_id}")
 
+    transaction_ids = {row.id for row in payload.transactions}
+    for s in payload.transaction_splits:
+        if s.transaction_id not in transaction_ids:
+            raise HTTPException(400, f"Transaction split {s.id} references unknown transaction_id {s.transaction_id}")
+        if s.category_id is not None and s.category_id not in category_ids:
+            raise HTTPException(400, f"Transaction split {s.id} references unknown category_id {s.category_id}")
+
     for v in payload.asset_valuations:
         if v.asset_id not in asset_ids:
             raise HTTPException(400, f"Asset valuation {v.id} references unknown asset_id {v.asset_id}")
@@ -176,8 +186,10 @@ async def restore_backup(session: AsyncSession, payload: BackupPayload) -> None:
         await session.execute(delete(GoalContribution))
         await session.execute(delete(Goal))
         await session.execute(delete(RecurringTransaction))
-        # Deleting transactions cascades transaction_tags rows (ON DELETE
-        # CASCADE) — tags themselves still need their own delete.
+        # Deleting transactions cascades transaction_tags and
+        # transaction_splits rows (ON DELETE CASCADE) — deleted explicitly
+        # here anyway to keep this block's ordering self-documenting.
+        await session.execute(delete(TransactionSplit))
         await session.execute(delete(Transaction))
         await session.execute(delete(Tag))
         await session.execute(delete(Asset))
@@ -208,6 +220,7 @@ async def restore_backup(session: AsyncSession, payload: BackupPayload) -> None:
             row.id: Transaction(**row.model_dump(exclude={"tag_ids"}), tags=[]) for row in payload.transactions
         }
         session.add_all(transactions_by_id.values())
+        session.add_all(TransactionSplit(**row.model_dump()) for row in payload.transaction_splits)
 
         session.add_all(AssetValuation(**row.model_dump()) for row in payload.asset_valuations)
         session.add_all(CryptoHolding(**row.model_dump()) for row in payload.crypto_holdings)
@@ -229,6 +242,7 @@ async def restore_backup(session: AsyncSession, payload: BackupPayload) -> None:
         await _reset_sequence(session, "tags", payload.tags)
         await _reset_sequence(session, "assets", payload.assets)
         await _reset_sequence(session, "transactions", payload.transactions)
+        await _reset_sequence(session, "transaction_splits", payload.transaction_splits)
         await _reset_sequence(session, "asset_valuations", payload.asset_valuations)
         await _reset_sequence(session, "crypto_transactions", payload.crypto_transactions)
         await _reset_sequence(session, "budgets", payload.budgets)

backend/app/services/budget_service.py

@@ -15,7 +15,7 @@ from sqlalchemy.orm import selectinload
 from app.models.budget import Budget
 from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
-from app.models.transaction import Transaction
+from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.budget import BudgetCreate, BudgetStatus, BudgetStatusResponse, BudgetUpdate
 
 _EAGER = (selectinload(Budget.category),)
@@ -92,7 +92,11 @@ async def get_budget_status(session: AsyncSession, year: int, month: int) -> Bud
     counted_ids = set(budget_category_ids).union(
         child_id for children in children_by_parent.values() for child_id in children
     )
-    spent_stmt = (
+    # A split transaction has category_id=NULL on its own row — its spend
+    # lives on its split lines instead (see category_rollup.py), so a plain
+    # sum on Transaction.category_id alone would silently under-count a
+    # budget funded partly by split purchases.
+    plain_stmt = (
         select(Transaction.category_id, func.coalesce(func.sum(Transaction.amount), 0))
         .where(
             Transaction.category_id.in_(counted_ids),
@@ -102,7 +106,22 @@ async def get_budget_status(session: AsyncSession, year: int, month: int) -> Bud
         )
         .group_by(Transaction.category_id)
     )
-    spent_by_category: dict[int, Decimal] = {row[0]: row[1] for row in (await session.execute(spent_stmt)).all()}
+    split_stmt = (
+        select(TransactionSplit.category_id, func.coalesce(func.sum(TransactionSplit.amount), 0))
+        .join(Transaction, Transaction.id == TransactionSplit.transaction_id)
+        .where(
+            TransactionSplit.category_id.in_(counted_ids),
+            Transaction.type == TransactionType.EXPENSE,
+            Transaction.date >= start,
+            Transaction.date <= end,
+        )
+        .group_by(TransactionSplit.category_id)
+    )
+    spent_by_category: dict[int, Decimal] = defaultdict(Decimal)
+    for category_id, amount in (await session.execute(plain_stmt)).all():
+        spent_by_category[category_id] += amount
+    for category_id, amount in (await session.execute(split_stmt)).all():
+        spent_by_category[category_id] += amount
 
     items = []
     for budget in budgets:

backend/app/services/category_rollup.py

@@ -0,0 +1,116 @@
+"""Spend/income per top-level category, split-aware — shared by
+dashboard_service's breakdown and reports_service's category ranking, the
+two places that both roll a subcategory's amount up into its parent
+(coalesce(parent_id, id)) and now also need a category's share of a split
+transaction counted the same way a plain transaction's is.
+
+A split transaction has Transaction.category_id=NULL (see
+models/transaction.py's TransactionSplit) — a query that only joins on that
+column would silently drop its spending from every category it actually
+touches. This walks both sources (a transaction's own category_id, and its
+splits' category_ids) and combines them before rolling up, so a category
+funded partly by plain transactions and partly by split lines still shows
+one correct total.
+"""
+from collections import defaultdict
+from dataclasses import dataclass
+from datetime import date as date_
+from decimal import Decimal
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.category import Category
+from app.models.enums import TransactionType
+from app.models.transaction import Transaction, TransactionSplit
+
+
+@dataclass
+class CategoryRollupItem:
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    sort_order: int
+    amount: Decimal
+    # Distinct transactions contributing to this category — a split
+    # transaction counts once per top-level category it touches, same as a
+    # plain one counts once for its own category.
+    transaction_count: int
+
+
+async def _raw_category_contributions(
+    session: AsyncSession,
+    *,
+    transaction_type: TransactionType,
+    start_date: date_ | None,
+    end_date: date_ | None,
+) -> list[tuple[int, int, Decimal]]:
+    """(transaction_id, category_id, amount) for every category a
+    transaction of this type/date-range contributed to. A plain transaction
+    contributes one row (its own category); a split one contributes one row
+    per split line — never both for the same transaction, since a
+    transaction is either plain (category_id set, no splits) or split
+    (category_id NULL, 2+ splits), enforced at write time."""
+    plain_stmt = select(Transaction.id, Transaction.category_id, Transaction.amount).where(
+        Transaction.type == transaction_type, Transaction.category_id.is_not(None)
+    )
+    split_stmt = (
+        select(TransactionSplit.transaction_id, TransactionSplit.category_id, TransactionSplit.amount)
+        .join(Transaction, Transaction.id == TransactionSplit.transaction_id)
+        .where(Transaction.type == transaction_type, TransactionSplit.category_id.is_not(None))
+    )
+    if start_date is not None:
+        plain_stmt = plain_stmt.where(Transaction.date >= start_date)
+        split_stmt = split_stmt.where(Transaction.date >= start_date)
+    if end_date is not None:
+        plain_stmt = plain_stmt.where(Transaction.date <= end_date)
+        split_stmt = split_stmt.where(Transaction.date <= end_date)
+
+    plain_rows = (await session.execute(plain_stmt)).all()
+    split_rows = (await session.execute(split_stmt)).all()
+    return [(r[0], r[1], r[2]) for r in plain_rows] + [(r[0], r[1], r[2]) for r in split_rows]
+
+
+async def rollup_spending_by_top_level_category(
+    session: AsyncSession,
+    *,
+    transaction_type: TransactionType,
+    start_date: date_ | None = None,
+    end_date: date_ | None = None,
+) -> list[CategoryRollupItem]:
+    """Every top-level category's total for the period, sorted by amount
+    desc (category sort_order as tiebreak — same order the SQL-only version
+    used to produce)."""
+    contributions = await _raw_category_contributions(
+        session, transaction_type=transaction_type, start_date=start_date, end_date=end_date
+    )
+    if not contributions:
+        return []
+
+    categories_by_id = {c.id: c for c in (await session.execute(select(Category))).scalars().all()}
+
+    amount_by_effective: dict[int, Decimal] = defaultdict(Decimal)
+    txn_ids_by_effective: dict[int, set[int]] = defaultdict(set)
+    for transaction_id, category_id, amount in contributions:
+        category = categories_by_id.get(category_id)
+        effective_id = category.parent_id if category and category.parent_id is not None else category_id
+        amount_by_effective[effective_id] += amount
+        txn_ids_by_effective[effective_id].add(transaction_id)
+
+    items: list[CategoryRollupItem] = []
+    for effective_id, amount in amount_by_effective.items():
+        category = categories_by_id.get(effective_id)
+        items.append(
+            CategoryRollupItem(
+                category_id=effective_id,
+                name=category.name if category else "?",
+                color=category.color if category else "#898781",
+                icon=category.icon if category else None,
+                sort_order=category.sort_order if category else 0,
+                amount=amount,
+                transaction_count=len(txn_ids_by_effective[effective_id]),
+            )
+        )
+    items.sort(key=lambda item: (-item.amount, item.sort_order))
+    return items

backend/app/services/dashboard_service.py

@@ -5,12 +5,11 @@ from decimal import Decimal
 
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy.orm import aliased
 
-from app.models.category import Category
 from app.models.enums import TransactionType
 from app.models.transaction import Transaction
 from app.schemas.dashboard import CategoryBreakdownItem, DashboardSummary
+from app.services.category_rollup import rollup_spending_by_top_level_category
 
 # Categorical slots are capped at 8 (dataviz skill: a 9th series folds into "Other",
 # never a generated hue) — this is also the exact size of the default category set.
@@ -38,36 +37,13 @@ async def get_dashboard_summary(session: AsyncSession, year: int, month: int) ->
     spent = totals.get(TransactionType.EXPENSE, Decimal("0"))
     transferred_out = totals.get(TransactionType.TRANSFER, Decimal("0"))
 
-    # A subcategory's spending rolls up into its parent's slice — Parent is a
-    # self-join to resolve each transaction's *top-level* category, falling
-    # back to the transaction's own category when it has no parent.
-    Parent = aliased(Category)
-    effective_id = func.coalesce(Category.parent_id, Category.id)
-    effective_name = func.coalesce(Parent.name, Category.name)
-    effective_color = func.coalesce(Parent.color, Category.color)
-    effective_icon = func.coalesce(Parent.icon, Category.icon)
-    effective_sort = func.coalesce(Parent.sort_order, Category.sort_order)
-
-    breakdown_stmt = (
-        select(
-            effective_id.label("category_id"),
-            effective_name.label("name"),
-            effective_color.label("color"),
-            effective_icon.label("icon"),
-            func.coalesce(func.sum(Transaction.amount), 0).label("amount"),
-        )
-        .join(Category, Category.id == Transaction.category_id)
-        .outerjoin(Parent, Parent.id == Category.parent_id)
-        .where(
-            Transaction.date >= start,
-            Transaction.date <= end,
-            Transaction.type == TransactionType.EXPENSE,
-        )
-        .group_by(effective_id, effective_name, effective_color, effective_icon, effective_sort)
-        .order_by(func.sum(Transaction.amount).desc(), effective_sort)
+    # A subcategory's spending rolls up into its parent's slice, and a split
+    # transaction's category_id=NULL means its category lives on its split
+    # lines instead — rollup_spending_by_top_level_category handles both
+    # the same way a plain transaction's category already was.
+    rows = await rollup_spending_by_top_level_category(
+        session, transaction_type=TransactionType.EXPENSE, start_date=start, end_date=end
     )
-    breakdown_result = await session.execute(breakdown_stmt)
-    rows = breakdown_result.all()
 
     top_rows, rest_rows = rows[:MAX_CHART_SLICES], rows[MAX_CHART_SLICES:]
 
@@ -76,9 +52,10 @@ async def get_dashboard_summary(session: AsyncSession, year: int, month: int) ->
 
     spending_by_category = [
         CategoryBreakdownItem(
-            category_id=cat_id, name=name, color=color, icon=icon, amount=amount, percent=_percent(amount)
+            category_id=row.category_id, name=row.name, color=row.color, icon=row.icon,
+            amount=row.amount, percent=_percent(row.amount),
         )
-        for cat_id, name, color, icon, amount in top_rows
+        for row in top_rows
     ]
 
     if rest_rows:

backend/app/services/reports_service.py

@@ -4,23 +4,24 @@ and how much in total over N years" (single-category detail), and "which
 categories cost the most over this whole period" (ranking, across all
 categories of one kind at once, not just the current month).
 """
+from collections import defaultdict
 from datetime import date as date_
 from decimal import Decimal
 
 from fastapi import HTTPException
-from sqlalchemy import extract, func, select
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy.orm import aliased
 
 from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
-from app.models.transaction import Transaction
+from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.reports import (
     CategoryRankingItem,
     CategoryRankingReport,
     CategorySpendingPoint,
     CategorySpendingReport,
 )
+from app.services.category_rollup import rollup_spending_by_top_level_category
 
 
 def _next_month(year: int, month: int) -> tuple[int, int]:
@@ -44,51 +45,56 @@ async def get_category_spending_report(
         ).scalars().all()
         category_ids.extend(child_ids)
 
-    bounds_stmt = select(func.min(Transaction.date), func.max(Transaction.date)).where(
+    # Plain transactions filed directly under one of these categories, plus
+    # split lines that assign part of a transaction to one of them — same
+    # two sources category_rollup.py unions for the Dashboard/ranking report.
+    plain_stmt = select(Transaction.id, Transaction.date, Transaction.amount).where(
         Transaction.category_id.in_(category_ids)
     )
+    split_stmt = (
+        select(TransactionSplit.transaction_id, Transaction.date, TransactionSplit.amount)
+        .join(Transaction, Transaction.id == TransactionSplit.transaction_id)
+        .where(TransactionSplit.category_id.in_(category_ids))
+    )
     if start_date:
-        bounds_stmt = bounds_stmt.where(Transaction.date >= start_date)
+        plain_stmt = plain_stmt.where(Transaction.date >= start_date)
+        split_stmt = split_stmt.where(Transaction.date >= start_date)
     if end_date:
-        bounds_stmt = bounds_stmt.where(Transaction.date <= end_date)
-    min_date, max_date = (await session.execute(bounds_stmt)).one()
+        plain_stmt = plain_stmt.where(Transaction.date <= end_date)
+        split_stmt = split_stmt.where(Transaction.date <= end_date)
 
-    effective_start = start_date or min_date
-    effective_end = end_date or max_date
+    plain_rows = (await session.execute(plain_stmt)).all()
+    split_rows = (await session.execute(split_stmt)).all()
+    contributions = [(r[0], r[1], r[2]) for r in plain_rows] + [(r[0], r[1], r[2]) for r in split_rows]
 
     empty = CategorySpendingReport(
         category_id=category.id,
         category_name=category.name,
         category_color=category.color,
         category_icon=category.icon,
-        start_date=effective_start,
-        end_date=effective_end,
+        start_date=start_date,
+        end_date=end_date,
         total_amount=Decimal("0"),
         transaction_count=0,
         average_per_month=Decimal("0"),
         series=[],
     )
-    if effective_start is None or effective_end is None:
+    if not contributions:
         return empty
 
-    rows_stmt = (
-        select(
-            extract("year", Transaction.date).label("year"),
-            extract("month", Transaction.date).label("month"),
-            func.sum(Transaction.amount).label("amount"),
-            func.count(Transaction.id).label("cnt"),
-        )
-        .where(
-            Transaction.category_id.in_(category_ids),
-            Transaction.date >= effective_start,
-            Transaction.date <= effective_end,
-        )
-        .group_by("year", "month")
-    )
-    rows = (await session.execute(rows_stmt)).all()
-    by_month: dict[tuple[int, int], Decimal] = {(int(row.year), int(row.month)): row.amount for row in rows}
-    total_amount = sum((row.amount for row in rows), Decimal("0"))
-    total_count = sum(row.cnt for row in rows)
+    dates = [txn_date for _, txn_date, _ in contributions]
+    effective_start = start_date or min(dates)
+    effective_end = end_date or max(dates)
+
+    by_month: dict[tuple[int, int], Decimal] = defaultdict(Decimal)
+    for _, txn_date, amount in contributions:
+        by_month[(txn_date.year, txn_date.month)] += amount
+
+    total_amount = sum((amount for _, _, amount in contributions), Decimal("0"))
+    # Distinct transactions, not rows — a transaction split across two of
+    # these categories (e.g. parent + one of its children) must count once,
+    # the same as a plain transaction filed under just one of them.
+    total_count = len({transaction_id for transaction_id, _, _ in contributions})
 
     series: list[CategorySpendingPoint] = []
     year, month = effective_start.year, effective_start.month
@@ -126,36 +132,15 @@ async def get_category_ranking_report(
     arbitrary period — "which category costs the most" across the whole
     range, unlike the month-scoped Dashboard breakdown or the
     single-category detail above."""
-    # Same subcategory-into-parent rollup as the Dashboard breakdown — see
-    # dashboard_service.get_dashboard_summary.
-    Parent = aliased(Category)
-    effective_id = func.coalesce(Category.parent_id, Category.id)
-    effective_name = func.coalesce(Parent.name, Category.name)
-    effective_color = func.coalesce(Parent.color, Category.color)
-    effective_icon = func.coalesce(Parent.icon, Category.icon)
-    effective_sort = func.coalesce(Parent.sort_order, Category.sort_order)
-
-    stmt = (
-        select(
-            effective_id.label("category_id"),
-            effective_name.label("name"),
-            effective_color.label("color"),
-            effective_icon.label("icon"),
-            func.sum(Transaction.amount).label("amount"),
-            func.count(Transaction.id).label("cnt"),
-        )
-        .join(Category, Category.id == Transaction.category_id)
-        .outerjoin(Parent, Parent.id == Category.parent_id)
-        .where(Category.kind == kind, Transaction.type == _KIND_TO_TRANSACTION_TYPE[kind])
+    # A transaction's type already restricts it to categories of the
+    # matching kind (enforced at write time by _ensure_category_matches_type
+    # in routes/transactions.py), so filtering by transaction_type below is
+    # enough — no separate kind filter needed, and the shared rollup already
+    # unions plain transactions with split lines the same way the Dashboard
+    # breakdown does.
+    rows = await rollup_spending_by_top_level_category(
+        session, transaction_type=_KIND_TO_TRANSACTION_TYPE[kind], start_date=start_date, end_date=end_date
     )
-    if start_date:
-        stmt = stmt.where(Transaction.date >= start_date)
-    if end_date:
-        stmt = stmt.where(Transaction.date <= end_date)
-    stmt = stmt.group_by(effective_id, effective_name, effective_color, effective_icon, effective_sort)
-    stmt = stmt.order_by(func.sum(Transaction.amount).desc(), effective_sort)
-
-    rows = (await session.execute(stmt)).all()
     total_amount = sum((row.amount for row in rows), Decimal("0"))
 
     def _percent(amount: Decimal) -> float:
@@ -163,17 +148,15 @@ async def get_category_ranking_report(
 
     items = [
         CategoryRankingItem(
-            category_id=cat_id,
-            name=name,
-            color=color,
-            icon=icon,
-            amount=amount,
-            percent=_percent(amount),
-            transaction_count=cnt,
+            category_id=row.category_id,
+            name=row.name,
+            color=row.color,
+            icon=row.icon,
+            amount=row.amount,
+            percent=_percent(row.amount),
+            transaction_count=row.transaction_count,
         )
-        for cat_id, name, color, icon, amount, cnt in rows
+        for row in rows
     ]
 
-    return CategoryRankingReport(
-        start_date=start_date, end_date=end_date, total_amount=total_amount, items=items
-    )
+    return CategoryRankingReport(start_date=start_date, end_date=end_date, total_amount=total_amount, items=items)

backend/tests/test_backup.py

@@ -62,3 +62,32 @@ async def test_backup_import_rejects_transaction_with_unknown_tag_id(client: Asy
 
     resp = await client.post("/backup/import", json=payload)
     assert resp.status_code == 400
+
+
+async def test_backup_roundtrip_preserves_transaction_splits(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    created = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00", "note": "household chemicals"},
+            ],
+        ),
+    )
+    txn_id = created.json()["id"]
+
+    payload = (await client.get("/backup/export")).json()
+    assert len(payload["transaction_splits"]) == 2
+
+    import_resp = await client.post("/backup/import", json=payload)
+    assert import_resp.status_code == 200, import_resp.text
+
+    refetched = next(t for t in (await client.get("/transactions")).json()["items"] if t["id"] == txn_id)
+    splits_by_note = {s["note"]: s for s in refetched["splits"]}
+    assert splits_by_note["household chemicals"]["category_id"] == shopping
+    assert refetched["category"] is None

backend/tests/test_budgets.py

@@ -132,3 +132,28 @@ async def test_a_subcategory_keeps_its_own_budget_separate(client: AsyncClient,
     assert items[child_id]["is_over_budget"] is True
     assert money(items[parent_id]["spent"]) == money("900.00")
     assert items[parent_id]["is_over_budget"] is False
+
+
+async def test_budget_status_counts_a_split_transactions_share(client: AsyncClient, account_id, categories):
+    """A receipt split between Groceries and Shopping must count its 70.00
+    slice toward the Groceries budget, not the full 100.00 or nothing."""
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    await client.post("/budgets", json={"category_id": groceries, "monthly_limit": "100.00"})
+    await client.post(
+        "/transactions",
+        json=txn_payload(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            date="2026-08-10",
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+
+    resp = await client.get("/budgets/status", params={"year": 2026, "month": 8})
+    item = resp.json()["items"][0]
+    assert money(item["spent"]) == Decimal("70.00")

backend/tests/test_dashboard.py

@@ -125,3 +125,34 @@ async def test_more_than_eight_expense_categories_roll_up_into_other(client: Asy
     # The 9th (smallest, index 8) category's 10.00 must be folded into Other,
     # not silently dropped from the total.
     assert money(other[0]["amount"]) == Decimal("10.00")
+
+
+async def test_split_transaction_counts_each_split_toward_its_own_category(
+    client: AsyncClient, account_id, categories
+):
+    """A hypermarket receipt split between Groceries and Shopping must show
+    up as two separate slices on the donut, not one lump under whichever
+    category happened to be picked."""
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            date="2026-08-01",
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+
+    resp = await client.get("/dashboard/summary", params={"year": 2026, "month": 8})
+    body = resp.json()
+    breakdown = {row["name"]: row for row in body["spending_by_category"]}
+
+    assert money(body["spent"]) == Decimal("100.00")
+    assert money(breakdown["Groceries"]["amount"]) == Decimal("70.00")
+    assert money(breakdown["Shopping"]["amount"]) == Decimal("30.00")

backend/tests/test_transactions.py

@@ -401,3 +401,227 @@ async def test_a_transfer_whose_destination_account_was_deleted_can_still_be_lis
     assert listing.status_code == 200
     assert txn_id in [item["id"] for item in listing.json()["items"]]
     assert (await client.delete(f"/transactions/{txn_id}")).status_code == 204
+
+
+async def test_create_split_transaction_across_categories(client: AsyncClient, account_id, categories):
+    """The hypermarket-receipt case: one purchase, part groceries, part
+    shopping — category_id stays empty on the row itself, and its splits
+    carry the categorization instead."""
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00", "note": "household chemicals"},
+            ],
+        ),
+    )
+    assert resp.status_code == 201
+    body = resp.json()
+    assert body["category"] is None
+    splits = {s["category"]["name"]: s for s in body["splits"]}
+    assert money(splits["Groceries"]["amount"]) == Decimal("70.00")
+    assert money(splits["Shopping"]["amount"]) == Decimal("30.00")
+    assert splits["Shopping"]["note"] == "household chemicals"
+
+
+async def test_create_split_rejects_category_id_alongside_splits(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=groceries,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+    assert resp.status_code == 422
+
+
+async def test_create_split_rejects_a_single_split(client: AsyncClient, account_id, categories):
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id, amount="100.00", category_id=None, splits=[{"category_id": categories["Groceries"]["id"], "amount": "100.00"}]
+        ),
+    )
+    assert resp.status_code == 422
+
+
+async def test_create_split_rejects_amounts_not_summing_to_total(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "20.00"},
+            ],
+        ),
+    )
+    assert resp.status_code == 422
+
+
+async def test_create_split_rejects_on_transfer(client: AsyncClient, account_id, categories):
+    other = (await client.post("/accounts", json={"name": "Savings", "type": "savings"})).json()
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            type="transfer",
+            amount="100.00",
+            category_id=None,
+            transfer_account_id=other["id"],
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+    assert resp.status_code == 422
+
+
+async def test_update_can_convert_a_transaction_into_a_split_and_back(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    created = await client.post("/transactions", json=_txn(account_id, amount="100.00", category_id=groceries))
+    txn_id = created.json()["id"]
+
+    split_resp = await client.patch(
+        f"/transactions/{txn_id}",
+        json={
+            "category_id": None,
+            "splits": [
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        },
+    )
+    assert split_resp.status_code == 200
+    assert len(split_resp.json()["splits"]) == 2
+    assert split_resp.json()["category"] is None
+
+    back_resp = await client.patch(f"/transactions/{txn_id}", json={"category_id": groceries, "splits": []})
+    assert back_resp.status_code == 200
+    assert back_resp.json()["splits"] == []
+    assert back_resp.json()["category"]["id"] == groceries
+
+
+async def test_update_rejects_splits_not_summing_to_the_new_amount(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    created = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+    txn_id = created.json()["id"]
+
+    # Splits weren't touched by this patch, but the new amount no longer
+    # matches their sum — the update must still be rejected.
+    resp = await client.patch(f"/transactions/{txn_id}", json={"amount": "50.00"})
+    assert resp.status_code == 400
+
+
+async def test_update_of_unrelated_field_tolerates_a_split_whose_category_was_deleted(
+    client: AsyncClient, account_id, categories
+):
+    """Same lesson as the transfer_account_id/deleted-account case: deleting
+    a category SET NULLs any split that pointed at it, and an edit to some
+    other field on the transaction must not be blocked by that pre-existing
+    state — the split's own category was never re-submitted, so nothing here
+    needs it to still exist."""
+    groceries = categories["Groceries"]["id"]
+    extra = await client.post(
+        "/categories", json={"name": "Deletable", "kind": "expense", "color": "#123456", "sort_order": 99}
+    )
+    extra_id = extra.json()["id"]
+    created = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": extra_id, "amount": "30.00"},
+            ],
+        ),
+    )
+    txn_id = created.json()["id"]
+    assert (await client.delete(f"/categories/{extra_id}")).status_code == 204
+
+    resp = await client.patch(f"/transactions/{txn_id}", json={"description": "renamed"})
+    assert resp.status_code == 200
+    assert resp.json()["description"] == "Renamed"
+    assert resp.json()["splits"][1]["category"] is None
+
+
+async def test_category_filter_matches_transactions_split_into_that_category(
+    client: AsyncClient, account_id, categories
+):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+
+    resp = await client.get("/transactions", params={"category_id": shopping})
+    assert resp.json()["total"] == 1
+
+
+async def test_delete_transaction_cascades_its_splits(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    created = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": shopping, "amount": "30.00"},
+            ],
+        ),
+    )
+    txn_id = created.json()["id"]
+
+    assert (await client.delete(f"/transactions/{txn_id}")).status_code == 204
+
+    # If the split rows outlived their parent, this budget/dashboard-style
+    # category filter would still find a (now-orphaned) match.
+    resp = await client.get("/transactions", params={"category_id": groceries})
+    assert resp.json()["total"] == 0

frontend/src/components/transactions/TransactionFormModal.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useState } from "react";
+import { Plus, X } from "lucide-react";
 import { Dialog } from "@/components/ui/Dialog";
 import { Button } from "@/components/ui/Button";
 import { Input, Label, Select } from "@/components/ui/Input";
@@ -8,7 +9,8 @@ import { useCategories } from "@/hooks/useCategories";
 import { useCreateTransaction, useUpdateTransaction } from "@/hooks/useTransactions";
 import { useTranslation } from "@/lib/i18n";
 import { translateCategoryName } from "@/lib/categoryLabels";
-import type { Category, Tag, Transaction, TransactionInput, TransactionType } from "@/types";
+import { formatCurrency } from "@/lib/format";
+import type { Category, Tag, Transaction, TransactionInput, TransactionSplitInput, TransactionType } from "@/types";
 
 interface TransactionFormModalProps {
   open: boolean;
@@ -32,6 +34,25 @@ const EMPTY_FORM = {
   date: todayIso(),
 };
 
+interface SplitRowState {
+  key: string;
+  category_id: string;
+  amount: string;
+  note: string;
+}
+
+function emptySplitRow(): SplitRowState {
+  return { key: crypto.randomUUID(), category_id: "", amount: "", note: "" };
+}
+
+// Cents, not floats — a plain Number sum of "0.10" + "0.20" style amounts can
+// drift from the transaction total by fractions of a cent, which would
+// falsely trip the "must add up exactly" check the backend also enforces.
+function toCents(value: string): number {
+  const parsed = Number(value);
+  return Number.isFinite(parsed) ? Math.round(parsed * 100) : 0;
+}
+
 export function TransactionFormModal({ open, onClose, transaction }: TransactionFormModalProps) {
   const { t, language } = useTranslation();
   const { data: accounts } = useAccounts();
@@ -41,6 +62,8 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
 
   const [form, setForm] = useState(EMPTY_FORM);
   const [tags, setTags] = useState<Tag[]>([]);
+  const [splitMode, setSplitMode] = useState(false);
+  const [splitRows, setSplitRows] = useState<SplitRowState[]>([emptySplitRow(), emptySplitRow()]);
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
@@ -58,9 +81,23 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
         date: transaction.date,
       });
       setTags(transaction.tags);
+      const hasSplits = transaction.splits.length > 0;
+      setSplitMode(hasSplits);
+      setSplitRows(
+        hasSplits
+          ? transaction.splits.map((split) => ({
+              key: String(split.id),
+              category_id: split.category_id ? String(split.category_id) : "",
+              amount: split.amount,
+              note: split.note ?? "",
+            }))
+          : [emptySplitRow(), emptySplitRow()]
+      );
     } else {
       setForm({ ...EMPTY_FORM, account_id: accounts?.[0] ? String(accounts[0].id) : "" });
       setTags([]);
+      setSplitMode(false);
+      setSplitRows([emptySplitRow(), emptySplitRow()]);
     }
     setError(null);
   }, [open, transaction, accounts]);
@@ -81,6 +118,22 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
 
   const isSaving = createTransaction.isPending || updateTransaction.isPending;
 
+  function updateSplitRow(key: string, patch: Partial<SplitRowState>) {
+    setSplitRows((prev) => prev.map((row) => (row.key === key ? { ...row, ...patch } : row)));
+  }
+
+  function addSplitRow() {
+    setSplitRows((prev) => [...prev, emptySplitRow()]);
+  }
+
+  function removeSplitRow(key: string) {
+    setSplitRows((prev) => (prev.length <= 2 ? prev : prev.filter((row) => row.key !== key)));
+  }
+
+  const isSplitEditingNow = form.type !== "transfer" && splitMode;
+  const splitAllocatedCents = splitRows.reduce((sum, row) => sum + toCents(row.amount), 0);
+  const splitRemainingCents = toCents(form.amount) - splitAllocatedCents;
+
   async function handleSubmit(event: React.FormEvent) {
     event.preventDefault();
     setError(null);
@@ -98,10 +151,44 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
       return;
     }
 
+    // Undefined -> leave the transaction's existing splits untouched on
+    // update, and create a normal single-category transaction. [] -> clears
+    // splits that used to be there (the user turned split mode off, or
+    // switched the transaction to a transfer, which can't carry splits).
+    let splits: TransactionSplitInput[] | undefined;
+    if (isSplitEditingNow) {
+      const filledRows = splitRows.filter((row) => row.category_id || row.amount);
+      if (filledRows.length < 2) {
+        setError(t("transactions.form.errorSplitMinRows"));
+        return;
+      }
+      if (filledRows.some((row) => !row.category_id || !row.amount || Number(row.amount) <= 0)) {
+        setError(t("transactions.form.errorSplitIncomplete"));
+        return;
+      }
+      const allocatedCents = filledRows.reduce((sum, row) => sum + toCents(row.amount), 0);
+      if (allocatedCents !== toCents(form.amount)) {
+        setError(t("transactions.form.errorSplitMismatch"));
+        return;
+      }
+      splits = filledRows.map((row) => ({
+        category_id: Number(row.category_id),
+        amount: row.amount,
+        note: row.note || null,
+      }));
+    } else if (transaction && transaction.splits.length > 0) {
+      splits = [];
+    }
+
     const payload: TransactionInput = {
       type: form.type,
       account_id: Number(form.account_id),
-      category_id: form.type === "transfer" ? null : form.category_id ? Number(form.category_id) : null,
+      category_id:
+        form.type === "transfer" || (splits && splits.length > 0)
+          ? null
+          : form.category_id
+            ? Number(form.category_id)
+            : null,
       transfer_account_id: form.type === "transfer" ? Number(form.transfer_account_id) : null,
       amount: form.amount,
       description: form.description,
@@ -109,6 +196,7 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
       notes: form.notes || null,
       date: form.date,
       tag_ids: tags.map((tag) => tag.id),
+      splits,
     };
 
     try {
@@ -135,9 +223,11 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
           <Select
             id="type"
             value={form.type}
-            onChange={(event) =>
-              setForm((prev) => ({ ...prev, type: event.target.value as TransactionType, category_id: "" }))
-            }
+            onChange={(event) => {
+              const nextType = event.target.value as TransactionType;
+              setForm((prev) => ({ ...prev, type: nextType, category_id: "" }));
+              if (nextType === "transfer") setSplitMode(false);
+            }}
           >
             <option value="expense">{t("transactions.form.typeExpense")}</option>
             <option value="income">{t("transactions.form.typeIncome")}</option>
@@ -223,20 +313,106 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
           </div>
         ) : (
           <div>
-            <Label htmlFor="category">{t("transactions.form.categoryLabel")}</Label>
-            <Select
-              id="category"
-              value={form.category_id}
-              onChange={(event) => setForm((prev) => ({ ...prev, category_id: event.target.value }))}
-            >
-              <option value="">{t("transactions.form.noCategory")}</option>
-              {relevantCategories.map((category) => (
-                <option key={category.id} value={category.id}>
-                  {category.indented ? `    ↳ ` : ""}
-                  {translateCategoryName(category.name)}
-                </option>
-              ))}
-            </Select>
+            <div className="flex items-center justify-between">
+              <Label htmlFor="category">{t("transactions.form.categoryLabel")}</Label>
+              <button
+                type="button"
+                className="mb-1 text-xs text-series-1 hover:underline"
+                onClick={() => setSplitMode((prev) => !prev)}
+              >
+                {splitMode ? t("transactions.form.splitToggleOff") : t("transactions.form.splitToggle")}
+              </button>
+            </div>
+
+            {!splitMode ? (
+              <Select
+                id="category"
+                value={form.category_id}
+                onChange={(event) => setForm((prev) => ({ ...prev, category_id: event.target.value }))}
+              >
+                <option value="">{t("transactions.form.noCategory")}</option>
+                {relevantCategories.map((category) => (
+                  <option key={category.id} value={category.id}>
+                    {category.indented ? `    ↳ ` : ""}
+                    {translateCategoryName(category.name)}
+                  </option>
+                ))}
+              </Select>
+            ) : (
+              <div className="space-y-2">
+                <p className="text-xs text-text-muted">{t("transactions.form.splitHint")}</p>
+                {splitRows.map((row) => (
+                  <div key={row.key} className="space-y-1.5 rounded-lg border border-border bg-surface-1 p-2">
+                    <div className="flex flex-col gap-1.5 sm:flex-row sm:items-center sm:gap-2">
+                      <Select
+                        aria-label={t("transactions.form.splitCategoryPlaceholder")}
+                        className="sm:flex-1"
+                        value={row.category_id}
+                        onChange={(event) => updateSplitRow(row.key, { category_id: event.target.value })}
+                      >
+                        <option value="" disabled>
+                          {t("transactions.form.splitCategoryPlaceholder")}
+                        </option>
+                        {relevantCategories.map((category) => (
+                          <option key={category.id} value={category.id}>
+                            {category.indented ? `    ↳ ` : ""}
+                            {translateCategoryName(category.name)}
+                          </option>
+                        ))}
+                      </Select>
+                      <div className="flex items-center gap-1.5">
+                        <Input
+                          type="number"
+                          step="0.01"
+                          min="0.01"
+                          className="w-24"
+                          placeholder={t("transactions.form.amountLabel")}
+                          value={row.amount}
+                          onChange={(event) => updateSplitRow(row.key, { amount: event.target.value })}
+                        />
+                        <button
+                          type="button"
+                          aria-label={t("transactions.form.splitRemoveRow")}
+                          onClick={() => removeSplitRow(row.key)}
+                          disabled={splitRows.length <= 2}
+                          className="shrink-0 rounded-md p-1.5 text-text-muted hover:bg-surface-2 hover:text-danger disabled:opacity-30"
+                        >
+                          <X size={15} />
+                        </button>
+                      </div>
+                    </div>
+                    <Input
+                      className="text-xs"
+                      placeholder={t("transactions.form.splitNotePlaceholder")}
+                      value={row.note}
+                      onChange={(event) => updateSplitRow(row.key, { note: event.target.value })}
+                    />
+                  </div>
+                ))}
+
+                <div className="flex items-center justify-between gap-2">
+                  <button
+                    type="button"
+                    onClick={addSplitRow}
+                    className="flex items-center gap-1 rounded-md py-1 text-xs text-series-1 hover:underline"
+                  >
+                    <Plus size={14} />
+                    {t("transactions.form.splitAddRow")}
+                  </button>
+                  <p className={`text-xs ${splitRemainingCents === 0 ? "text-success" : "text-text-muted"}`}>
+                    {splitRemainingCents > 0
+                      ? t("transactions.form.splitRemainingLabel", {
+                          amount: formatCurrency(splitRemainingCents / 100),
+                        })
+                      : splitRemainingCents < 0
+                        ? t("transactions.form.splitOverAllocatedLabel", {
+                            amount: formatCurrency(Math.abs(splitRemainingCents) / 100),
+                          })
+                        : t("transactions.form.splitFullyAllocatedLabel")}
+                  </p>
+                </div>
+              </div>
+            )}
           </div>
         )}
 

frontend/src/components/transactions/TransactionsTable.tsx

@@ -1,5 +1,5 @@
 import { useState } from "react";
-import { ArrowLeftRight, CalendarSearch, Pencil, StickyNote, Trash2 } from "lucide-react";
+import { ArrowLeftRight, CalendarSearch, Pencil, SquareDivide, StickyNote, Trash2 } from "lucide-react";
 import { Dialog } from "@/components/ui/Dialog";
 import { getCategoryIcon } from "@/lib/icons";
 import { formatCurrency, formatTransactionDate } from "@/lib/format";
@@ -35,8 +35,14 @@ export function TransactionsTable({ items, onEdit, onDelete, onJumpToMonth }: Tr
         {items.map((tx) => {
           const isTransfer = tx.type === "transfer";
           const isExpense = tx.type === "expense";
-          const Icon = isTransfer ? ArrowLeftRight : getCategoryIcon(tx.category?.icon);
-          const color = isTransfer ? "var(--text-muted)" : tx.category?.color ?? "var(--text-muted)";
+          const isSplit = tx.splits.length > 0;
+          const Icon = isTransfer ? ArrowLeftRight : isSplit ? SquareDivide : getCategoryIcon(tx.category?.icon);
+          const color = isTransfer || isSplit ? "var(--text-muted)" : tx.category?.color ?? "var(--text-muted)";
+          const categoryLabel = isSplit
+            ? tx.splits.map((split) => (split.category ? translateCategoryName(split.category.name) : "?")).join(" + ")
+            : tx.category
+              ? translateCategoryName(tx.category.name)
+              : null;
 
           return (
             <li key={tx.id} className="group flex items-center gap-3 py-3">
@@ -52,7 +58,7 @@ export function TransactionsTable({ items, onEdit, onDelete, onJumpToMonth }: Tr
                 <span className="block truncate text-xs text-text-muted">
                   {formatTransactionDate(tx.date, Boolean(onJumpToMonth))} · {tx.account.name}
                   {isTransfer && tx.transfer_account_id ? ` ${t("transactions.transferSuffix")}` : ""}
-                  {tx.category ? ` · ${translateCategoryName(tx.category.name)}` : ""}
+                  {categoryLabel ? ` · ${categoryLabel}` : ""}
                   {tx.tags.length > 0 ? ` · ${tx.tags.map((tag) => tag.name).join(", ")}` : ""}
                 </span>
               </span>

frontend/src/lib/i18n.ts

@@ -272,6 +272,19 @@ const ru = {
   "transactions.form.errorSelectDestination": "Выберите счёт назначения для перевода",
   "transactions.form.errorSameAccount": "Счёт назначения должен отличаться от исходного счёта",
   "transactions.form.saveError": "Не удалось сохранить транзакцию. Проверьте данные и попробуйте снова.",
+  "transactions.form.splitToggle": "Разбить на несколько категорий",
+  "transactions.form.splitToggleOff": "Одна категория",
+  "transactions.form.splitHint": "Одна покупка — несколько категорий, например часть чека — продукты, часть — бытовая химия.",
+  "transactions.form.splitCategoryPlaceholder": "Выберите категорию",
+  "transactions.form.splitNotePlaceholder": "Заметка (необязательно)",
+  "transactions.form.splitAddRow": "Добавить категорию",
+  "transactions.form.splitRemoveRow": "Убрать категорию",
+  "transactions.form.splitRemainingLabel": "Осталось распределить: {{amount}}",
+  "transactions.form.splitOverAllocatedLabel": "Превышение суммы на: {{amount}}",
+  "transactions.form.splitFullyAllocatedLabel": "Вся сумма распределена",
+  "transactions.form.errorSplitMinRows": "Нужно как минимум 2 категории для разбивки",
+  "transactions.form.errorSplitIncomplete": "Выберите категорию и сумму для каждой строки разбивки",
+  "transactions.form.errorSplitMismatch": "Сумма по категориям должна совпадать с суммой транзакции",
 
   "reports.categoryLabel": "Категория",
   "reports.expenseGroup": "Расходы",
@@ -734,6 +747,19 @@ const en: Record<keyof typeof ru, string> = {
   "transactions.form.errorSelectDestination": "Select a destination account for the transfer",
   "transactions.form.errorSameAccount": "The destination account must be different from the source account",
   "transactions.form.saveError": "Failed to save the transaction. Check the data and try again.",
+  "transactions.form.splitToggle": "Split across multiple categories",
+  "transactions.form.splitToggleOff": "Single category",
+  "transactions.form.splitHint": "One purchase, several categories — e.g. part of a receipt is groceries, part is household chemicals.",
+  "transactions.form.splitCategoryPlaceholder": "Select a category",
+  "transactions.form.splitNotePlaceholder": "Note (optional)",
+  "transactions.form.splitAddRow": "Add category",
+  "transactions.form.splitRemoveRow": "Remove category",
+  "transactions.form.splitRemainingLabel": "Remaining to allocate: {{amount}}",
+  "transactions.form.splitOverAllocatedLabel": "Over-allocated by: {{amount}}",
+  "transactions.form.splitFullyAllocatedLabel": "Fully allocated",
+  "transactions.form.errorSplitMinRows": "Splitting needs at least 2 categories",
+  "transactions.form.errorSplitIncomplete": "Pick a category and an amount for every split row",
+  "transactions.form.errorSplitMismatch": "Split amounts must add up to the transaction amount",
 
   "reports.categoryLabel": "Category",
   "reports.expenseGroup": "Expenses",

frontend/src/types/index.ts

@@ -62,6 +62,25 @@ export interface Tag {
   name: string;
 }
 
+// One category's slice of a transaction whose amount is divided across
+// several categories (one receipt, several kinds of goods) — see
+// TransactionInput.splits. category is null when the split's category was
+// since deleted; reading must still work even though creating/editing a
+// split always requires a live category.
+export interface TransactionSplit {
+  id: number;
+  category_id: number | null;
+  category: Category | null;
+  amount: string;
+  note: string | null;
+}
+
+export interface TransactionSplitInput {
+  category_id: number;
+  amount: string;
+  note?: string | null;
+}
+
 export interface Transaction {
   id: number;
   account_id: number;
@@ -76,6 +95,7 @@ export interface Transaction {
   account: Account;
   category: Category | null;
   tags: Tag[];
+  splits: TransactionSplit[];
 }
 
 export interface TransactionPage {
@@ -98,6 +118,11 @@ export interface TransactionInput {
   // Omitted -> tags untouched on update; sent (even as []) -> replaces the
   // full tag set. Always sent on create (defaults to []).
   tag_ids?: number[];
+  // Omitted -> a normal single-category transaction (unchanged). 2+ entries
+  // -> the amount is divided across categories instead, and category_id
+  // above must then be null. On update, omitted leaves existing splits
+  // untouched; sent (even as []) replaces the full split set.
+  splits?: TransactionSplitInput[] | null;
 }
 
 export interface RecurringTransaction {