fix: restrict transaction splits to one parent category, show subcategory breakdown

Раньше сплит позволял смешивать вообще любые категории. Реальная задача —
разложить одну покупку по подкатегориям ОДНОЙ базовой категории (100 € на
продукты: сколько на сладости, сколько на уход). А раз Dashboard и рейтинг
категорий сворачивают подкатегории в родителя, эта разбивка нигде не была
видна.

- _build_splits() требует, чтобы все категории сплита были одной родительской
  категорией или её прямыми подкатегориями.
- category_rollup.py отдаёт для каждого родительского тотала список
  category-источников, которые в него скинулись (children) — пусто, если
  источник один.
- Dashboard и рейтинг категорий: строка с непустым children раскрывается по
  клику, показывая вклад каждой подкатегории.
- Форма транзакции: выбор категории при сплите — сначала базовая (верхний
  уровень), затем строки сплита выбирают саму базовую или её подкатегорию.

backend/app/api/routes/transactions.py

@@ -52,13 +52,15 @@ _TYPE_TO_CATEGORY_KIND = {
 
 async def _ensure_category_matches_type(
     session: AsyncSession, category_id: int | None, transaction_type: TransactionType
-) -> None:
+) -> Category | None:
     """A category picked for an income transaction must itself be an income
     category (and likewise for expense) — otherwise the dashboard's spending
     breakdown, which only joins EXPENSE-typed rows, would silently misclassify
-    the entry."""
+    the entry. Returns the fetched category (or None for category_id=None) so
+    callers that also need the row itself — _build_splits, below — don't have
+    to fetch it a second time."""
     if category_id is None:
-        return
+        return None
     expected_kind = _TYPE_TO_CATEGORY_KIND.get(transaction_type)
     category = await session.get(Category, category_id)
     if category is None:
@@ -68,13 +70,32 @@ async def _ensure_category_matches_type(
             status_code=400,
             detail=f"Category '{category.name}' is a {category.kind.value} category and cannot be used for a {transaction_type.value} transaction",
         )
+    return category
 
 
 async def _build_splits(
     session: AsyncSession, splits: list[TransactionSplitInput], transaction_type: TransactionType
 ) -> list[TransactionSplit]:
+    """A split's whole point is dividing one purchase's total across the
+    *subcategories of one parent* (a hypermarket receipt: part groceries ->
+    Sweets, part -> Alcohol) — not across unrelated top-level categories, or
+    the numbers would roll up into two different parents and the "spent X on
+    Groceries, split between Sweets/Alcohol" picture the feature exists for
+    falls apart. Each split may point at that parent category itself (an
+    unspecified-subcategory line) or at any one of its direct children —
+    enforced by requiring every split's own top-level ancestor
+    (parent_id, or its own id if it has none) to agree.
+    """
+    top_level_ids: set[int] = set()
     for split in splits:
-        await _ensure_category_matches_type(session, split.category_id, transaction_type)
+        category = await _ensure_category_matches_type(session, split.category_id, transaction_type)
+        assert category is not None  # split.category_id is required (not Optional) on the schema
+        top_level_ids.add(category.parent_id if category.parent_id is not None else category.id)
+    if len(top_level_ids) > 1:
+        raise HTTPException(
+            status_code=400,
+            detail="All split categories must be the same parent category or its direct subcategories",
+        )
     return [TransactionSplit(category_id=s.category_id, amount=s.amount, note=s.note) for s in splits]
 
 
@@ -196,17 +217,13 @@ 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", "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
-            ]
+            transaction.splits = await _build_splits(session, item.splits, item.type)
         transactions.append(transaction)
 
     session.add_all(transactions)

backend/app/schemas/dashboard.py

@@ -1,6 +1,18 @@
 from decimal import Decimal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+
+class CategoryBreakdownChildItem(BaseModel):
+    """One subcategory's (or the parent's own direct, un-subcategorized)
+    share of a CategoryBreakdownItem's total — see
+    services/category_rollup.py's CategoryRollupChildItem."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
 
 
 class CategoryBreakdownItem(BaseModel):
@@ -10,6 +22,10 @@ class CategoryBreakdownItem(BaseModel):
     icon: str | None
     amount: Decimal
     percent: float
+    # Populated only when this slice's spend came from more than one
+    # distinct category (subcategories, or a mix of the parent itself and
+    # its children) — e.g. a receipt split across "Groceries" subcategories.
+    children: list[CategoryBreakdownChildItem] = Field(default_factory=list)
 
 
 class DashboardSummary(BaseModel):

backend/app/schemas/reports.py

@@ -1,7 +1,19 @@
 from datetime import date as date_
 from decimal import Decimal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+
+class CategoryRankingChildItem(BaseModel):
+    """One subcategory's (or the parent's own direct, un-subcategorized)
+    share of a CategoryRankingItem's total — see
+    services/category_rollup.py's CategoryRollupChildItem."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
 
 
 class CategorySpendingPoint(BaseModel):
@@ -31,6 +43,10 @@ class CategoryRankingItem(BaseModel):
     amount: Decimal
     percent: float
     transaction_count: int
+    # Populated only when this category's spend came from more than one
+    # distinct category (subcategories, or a mix of itself and its
+    # children) — e.g. a receipt split across "Groceries" subcategories.
+    children: list[CategoryRankingChildItem] = Field(default_factory=list)
 
 
 class CategoryRankingReport(BaseModel):

backend/app/services/category_rollup.py

@@ -13,7 +13,7 @@ funded partly by plain transactions and partly by split lines still shows
 one correct total.
 """
 from collections import defaultdict
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from datetime import date as date_
 from decimal import Decimal
 
@@ -25,6 +25,20 @@ from app.models.enums import TransactionType
 from app.models.transaction import Transaction, TransactionSplit
 
 
+@dataclass
+class CategoryRollupChildItem:
+    """One leaf category's share of its parent's total — a real
+    subcategory, or the parent category itself when some of the spend was
+    filed directly under it with no more specific subcategory (category_id
+    equals the enclosing CategoryRollupItem.category_id in that case)."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
+
+
 @dataclass
 class CategoryRollupItem:
     category_id: int
@@ -37,6 +51,13 @@ class CategoryRollupItem:
     # transaction counts once per top-level category it touches, same as a
     # plain one counts once for its own category.
     transaction_count: int
+    # Populated only when more than one distinct category actually fed this
+    # total (a subcategory, or a direct-to-parent line, or several of
+    # either) — e.g. a receipt split between "Sweets" and "Alcohol" under
+    # "Groceries". A single-source category (the common case, no splitting
+    # or subcategories involved) leaves this empty rather than showing a
+    # redundant one-item breakdown of itself.
+    children: list[CategoryRollupChildItem] = field(default_factory=list)
 
 
 async def _raw_category_contributions(
@@ -92,15 +113,34 @@ async def rollup_spending_by_top_level_category(
 
     amount_by_effective: dict[int, Decimal] = defaultdict(Decimal)
     txn_ids_by_effective: dict[int, set[int]] = defaultdict(set)
+    # effective (top-level) category id -> {actual leaf category_id: amount}
+    # — the leaf is the same as the effective id when a contribution was
+    # filed directly on the parent, and a genuine child id otherwise.
+    amount_by_leaf: dict[int, dict[int, Decimal]] = defaultdict(lambda: defaultdict(Decimal))
     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
+        amount_by_leaf[effective_id][category_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)
+        leaf_amounts = amount_by_leaf[effective_id]
+        children: list[CategoryRollupChildItem] = []
+        if len(leaf_amounts) > 1:
+            for leaf_id, leaf_amount in sorted(leaf_amounts.items(), key=lambda pair: -pair[1]):
+                leaf_category = categories_by_id.get(leaf_id)
+                children.append(
+                    CategoryRollupChildItem(
+                        category_id=leaf_id,
+                        name=leaf_category.name if leaf_category else "?",
+                        color=leaf_category.color if leaf_category else "#898781",
+                        icon=leaf_category.icon if leaf_category else None,
+                        amount=leaf_amount,
+                    )
+                )
         items.append(
             CategoryRollupItem(
                 category_id=effective_id,
@@ -110,6 +150,7 @@ async def rollup_spending_by_top_level_category(
                 sort_order=category.sort_order if category else 0,
                 amount=amount,
                 transaction_count=len(txn_ids_by_effective[effective_id]),
+                children=children,
             )
         )
     items.sort(key=lambda item: (-item.amount, item.sort_order))

backend/app/services/dashboard_service.py

@@ -8,7 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.enums import TransactionType
 from app.models.transaction import Transaction
-from app.schemas.dashboard import CategoryBreakdownItem, DashboardSummary
+from app.schemas.dashboard import CategoryBreakdownChildItem, 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",
@@ -54,6 +54,13 @@ async def get_dashboard_summary(session: AsyncSession, year: int, month: int) ->
         CategoryBreakdownItem(
             category_id=row.category_id, name=row.name, color=row.color, icon=row.icon,
             amount=row.amount, percent=_percent(row.amount),
+            children=[
+                CategoryBreakdownChildItem(
+                    category_id=child.category_id, name=child.name, color=child.color, icon=child.icon,
+                    amount=child.amount,
+                )
+                for child in row.children
+            ],
         )
         for row in top_rows
     ]

backend/app/services/reports_service.py

@@ -16,6 +16,7 @@ from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
 from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.reports import (
+    CategoryRankingChildItem,
     CategoryRankingItem,
     CategoryRankingReport,
     CategorySpendingPoint,
@@ -155,6 +156,13 @@ async def get_category_ranking_report(
             amount=row.amount,
             percent=_percent(row.amount),
             transaction_count=row.transaction_count,
+            children=[
+                CategoryRankingChildItem(
+                    category_id=child.category_id, name=child.name, color=child.color, icon=child.icon,
+                    amount=child.amount,
+                )
+                for child in row.children
+            ],
         )
         for row in rows
     ]

backend/tests/test_backup.py

@@ -66,7 +66,9 @@ async def test_backup_import_rejects_transaction_with_unknown_tag_id(client: Asy
 
 async def test_backup_roundtrip_preserves_transaction_splits(client: AsyncClient, account_id, categories):
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets = (
+        await client.post("/categories", json={"name": "Sweets", "kind": "expense", "color": "#7a869a", "parent_id": groceries})
+    ).json()["id"]
     created = await client.post(
         "/transactions",
         json=_txn(
@@ -75,7 +77,7 @@ async def test_backup_roundtrip_preserves_transaction_splits(client: AsyncClient
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00", "note": "household chemicals"},
+                {"category_id": sweets, "amount": "30.00", "note": "candy and snacks"},
             ],
         ),
     )
@@ -89,5 +91,5 @@ async def test_backup_roundtrip_preserves_transaction_splits(client: AsyncClient
 
     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 splits_by_note["candy and snacks"]["category_id"] == sweets
     assert refetched["category"] is None

backend/tests/test_budgets.py

@@ -135,11 +135,17 @@ async def test_a_subcategory_keeps_its_own_budget_separate(client: AsyncClient,
 
 
 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."""
+    """A receipt split between Groceries and one of its own subcategories
+    (Sweets) must count each line toward the right budget: the full 100.00
+    rolls up into the Groceries budget, but only the 30.00 Sweets line
+    counts toward a budget set on Sweets itself."""
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets_resp = await client.post(
+        "/categories", json={"name": "Sweets", "kind": "expense", "color": "#7a869a", "parent_id": groceries}
+    )
+    sweets = sweets_resp.json()["id"]
     await client.post("/budgets", json={"category_id": groceries, "monthly_limit": "100.00"})
+    await client.post("/budgets", json={"category_id": sweets, "monthly_limit": "50.00"})
     await client.post(
         "/transactions",
         json=txn_payload(
@@ -149,11 +155,12 @@ async def test_budget_status_counts_a_split_transactions_share(client: AsyncClie
             date="2026-08-10",
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "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")
+    items = {item["category_id"]: item for item in resp.json()["items"]}
+    assert money(items[groceries]["spent"]) == Decimal("100.00")
+    assert money(items[sweets]["spent"]) == Decimal("30.00")

backend/tests/test_dashboard.py

@@ -127,14 +127,20 @@ async def test_more_than_eight_expense_categories_roll_up_into_other(client: Asy
     assert money(other[0]["amount"]) == Decimal("10.00")
 
 
-async def test_split_transaction_counts_each_split_toward_its_own_category(
+async def test_split_transaction_rolls_up_into_one_slice_with_a_children_breakdown(
     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."""
+    """A hypermarket receipt split between Groceries and one of its own
+    subcategories (Sweets) must show up as a single Groceries slice on the
+    donut with the full 100.00 total — a split's categories always share one
+    parent (see routes/transactions.py's _build_splits), so it never
+    fragments into separate top-level slices — but the slice must expose a
+    children breakdown so the Groceries/Sweets split is still visible."""
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets_resp = await client.post(
+        "/categories", json={"name": "Sweets", "kind": "expense", "color": "#7a869a", "parent_id": groceries}
+    )
+    sweets = sweets_resp.json()["id"]
     await client.post(
         "/transactions",
         json=_txn(
@@ -144,7 +150,7 @@ async def test_split_transaction_counts_each_split_toward_its_own_category(
             date="2026-08-01",
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
@@ -154,5 +160,25 @@ async def test_split_transaction_counts_each_split_toward_its_own_category(
     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")
+    assert "Sweets" not in breakdown
+    assert money(breakdown["Groceries"]["amount"]) == Decimal("100.00")
+
+    children = {child["name"]: child for child in breakdown["Groceries"]["children"]}
+    # The 70.00 line was filed directly on Groceries itself (no
+    # subcategory), so it shows up as a child entry with Groceries' own id
+    # and name too — same as any genuine subcategory would.
+    assert money(children["Groceries"]["amount"]) == Decimal("70.00")
+    assert money(children["Sweets"]["amount"]) == Decimal("30.00")
+
+
+async def test_a_single_category_slice_has_no_children_breakdown(client: AsyncClient, account_id, categories):
+    """A category funded by exactly one source (the common case — no
+    splitting, no subcategories involved) shouldn't show a redundant
+    one-item breakdown of itself."""
+    groceries = categories["Groceries"]["id"]
+    await client.post("/transactions", json=_txn(account_id, amount="50.00", category_id=groceries, date="2026-08-01"))
+
+    resp = await client.get("/dashboard/summary", params={"year": 2026, "month": 8})
+    breakdown = {row["name"]: row for row in resp.json()["spending_by_category"]}
+
+    assert breakdown["Groceries"]["children"] == []

backend/tests/test_reports.py

@@ -0,0 +1,78 @@
+"""Long-range reports: the category ranking (which categories cost the
+most over an arbitrary period) and the single-category spending detail —
+both split-aware via services/category_rollup.py, same as the Dashboard
+breakdown (see test_dashboard.py).
+"""
+from decimal import Decimal
+
+from httpx import AsyncClient
+
+from tests.helpers import money, txn_payload as _txn
+
+
+async def test_ranking_rolls_up_a_split_into_one_item_with_children(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    sweets = (
+        await client.post("/categories", json={"name": "Sweets", "kind": "expense", "color": "#7a869a", "parent_id": groceries})
+    ).json()["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": sweets, "amount": "30.00"},
+            ],
+        ),
+    )
+
+    resp = await client.get("/reports/category-ranking", params={"kind": "expense"})
+    items = {item["name"]: item for item in resp.json()["items"]}
+
+    assert "Sweets" not in items
+    assert money(items["Groceries"]["amount"]) == Decimal("100.00")
+    children = {child["name"]: child for child in items["Groceries"]["children"]}
+    assert money(children["Groceries"]["amount"]) == Decimal("70.00")
+    assert money(children["Sweets"]["amount"]) == Decimal("30.00")
+
+
+async def test_ranking_item_with_a_single_source_has_no_children(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    await client.post("/transactions", json=_txn(account_id, amount="50.00", category_id=groceries, date="2026-08-01"))
+
+    resp = await client.get("/reports/category-ranking", params={"kind": "expense"})
+    items = {item["name"]: item for item in resp.json()["items"]}
+
+    assert items["Groceries"]["children"] == []
+
+
+async def test_category_spending_report_counts_a_subcategorys_split_share(client: AsyncClient, account_id, categories):
+    """A top-level category's own report folds in both its plain
+    transactions and any split lines landing on it or its subcategories."""
+    groceries = categories["Groceries"]["id"]
+    sweets = (
+        await client.post("/categories", json={"name": "Sweets", "kind": "expense", "color": "#7a869a", "parent_id": groceries})
+    ).json()["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": sweets, "amount": "30.00"},
+            ],
+        ),
+    )
+
+    parent_report = (await client.get("/reports/category-spending", params={"category_id": groceries})).json()
+    assert money(parent_report["total_amount"]) == Decimal("100.00")
+    assert parent_report["transaction_count"] == 1
+
+    child_report = (await client.get("/reports/category-spending", params={"category_id": sweets})).json()
+    assert money(child_report["total_amount"]) == Decimal("30.00")

backend/tests/test_transactions.py

@@ -403,10 +403,47 @@ async def test_a_transfer_whose_destination_account_was_deleted_can_still_be_lis
     assert (await client.delete(f"/transactions/{txn_id}")).status_code == 204
 
 
+async def _subcategory(client: AsyncClient, parent_id: int, name: str) -> int:
+    resp = await client.post(
+        "/categories", json={"name": name, "kind": "expense", "color": "#7a869a", "parent_id": parent_id}
+    )
+    return resp.json()["id"]
+
+
 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."""
+    """The hypermarket-receipt case: one purchase, part everyday groceries,
+    part sweets — a split's categories must share one parent (see
+    _build_splits), so both lines here are Groceries itself and one of its
+    own subcategories. category_id stays empty on the row itself, and its
+    splits carry the categorization instead."""
+    groceries = categories["Groceries"]["id"]
+    sweets = await _subcategory(client, groceries, "Sweets")
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": groceries, "amount": "70.00"},
+                {"category_id": sweets, "amount": "30.00", "note": "candy and snacks"},
+            ],
+        ),
+    )
+    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["Sweets"]["amount"]) == Decimal("30.00")
+    assert splits["Sweets"]["note"] == "candy and snacks"
+
+
+async def test_create_split_rejects_categories_from_different_parents(client: AsyncClient, account_id, categories):
+    """The whole point of a split is dividing one purchase across the
+    subcategories of ONE parent — Groceries and Shopping are two unrelated
+    top-level categories, so mixing them must be rejected even though both
+    are individually valid expense categories."""
     groceries = categories["Groceries"]["id"]
     shopping = categories["Shopping"]["id"]
     resp = await client.post(
@@ -417,22 +454,36 @@ async def test_create_split_transaction_across_categories(client: AsyncClient, a
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00", "note": "household chemicals"},
+                {"category_id": shopping, "amount": "30.00"},
             ],
         ),
     )
-    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"
+    assert resp.status_code == 400
+
+
+async def test_create_split_rejects_two_different_parents_subcategories(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    shopping = categories["Shopping"]["id"]
+    sweets = await _subcategory(client, groceries, "Sweets")
+    electronics = await _subcategory(client, shopping, "Electronics")
+    resp = await client.post(
+        "/transactions",
+        json=_txn(
+            account_id,
+            amount="100.00",
+            category_id=None,
+            splits=[
+                {"category_id": sweets, "amount": "70.00"},
+                {"category_id": electronics, "amount": "30.00"},
+            ],
+        ),
+    )
+    assert resp.status_code == 400
 
 
 async def test_create_split_rejects_category_id_alongside_splits(client: AsyncClient, account_id, categories):
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     resp = await client.post(
         "/transactions",
         json=_txn(
@@ -441,7 +492,7 @@ async def test_create_split_rejects_category_id_alongside_splits(client: AsyncCl
             category_id=groceries,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
@@ -460,7 +511,7 @@ async def test_create_split_rejects_a_single_split(client: AsyncClient, account_
 
 async def test_create_split_rejects_amounts_not_summing_to_total(client: AsyncClient, account_id, categories):
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     resp = await client.post(
         "/transactions",
         json=_txn(
@@ -469,7 +520,7 @@ async def test_create_split_rejects_amounts_not_summing_to_total(client: AsyncCl
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "20.00"},
+                {"category_id": sweets, "amount": "20.00"},
             ],
         ),
     )
@@ -479,7 +530,7 @@ async def test_create_split_rejects_amounts_not_summing_to_total(client: AsyncCl
 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"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     resp = await client.post(
         "/transactions",
         json=_txn(
@@ -490,7 +541,7 @@ async def test_create_split_rejects_on_transfer(client: AsyncClient, account_id,
             transfer_account_id=other["id"],
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
@@ -499,7 +550,7 @@ async def test_create_split_rejects_on_transfer(client: AsyncClient, account_id,
 
 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"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     created = await client.post("/transactions", json=_txn(account_id, amount="100.00", category_id=groceries))
     txn_id = created.json()["id"]
 
@@ -509,7 +560,7 @@ async def test_update_can_convert_a_transaction_into_a_split_and_back(client: As
             "category_id": None,
             "splits": [
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         },
     )
@@ -525,7 +576,7 @@ async def test_update_can_convert_a_transaction_into_a_split_and_back(client: As
 
 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"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     created = await client.post(
         "/transactions",
         json=_txn(
@@ -534,7 +585,7 @@ async def test_update_rejects_splits_not_summing_to_the_new_amount(client: Async
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
@@ -555,10 +606,7 @@ async def test_update_of_unrelated_field_tolerates_a_split_whose_category_was_de
     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"]
+    deletable = await _subcategory(client, groceries, "Deletable")
     created = await client.post(
         "/transactions",
         json=_txn(
@@ -567,12 +615,12 @@ async def test_update_of_unrelated_field_tolerates_a_split_whose_category_was_de
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": extra_id, "amount": "30.00"},
+                {"category_id": deletable, "amount": "30.00"},
             ],
         ),
     )
     txn_id = created.json()["id"]
-    assert (await client.delete(f"/categories/{extra_id}")).status_code == 204
+    assert (await client.delete(f"/categories/{deletable}")).status_code == 204
 
     resp = await client.patch(f"/transactions/{txn_id}", json={"description": "renamed"})
     assert resp.status_code == 200
@@ -584,7 +632,7 @@ async def test_category_filter_matches_transactions_split_into_that_category(
     client: AsyncClient, account_id, categories
 ):
     groceries = categories["Groceries"]["id"]
-    shopping = categories["Shopping"]["id"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     await client.post(
         "/transactions",
         json=_txn(
@@ -593,18 +641,18 @@ async def test_category_filter_matches_transactions_split_into_that_category(
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
 
-    resp = await client.get("/transactions", params={"category_id": shopping})
+    resp = await client.get("/transactions", params={"category_id": sweets})
     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"]
+    sweets = await _subcategory(client, groceries, "Sweets")
     created = await client.post(
         "/transactions",
         json=_txn(
@@ -613,7 +661,7 @@ async def test_delete_transaction_cascades_its_splits(client: AsyncClient, accou
             category_id=None,
             splits=[
                 {"category_id": groceries, "amount": "70.00"},
-                {"category_id": shopping, "amount": "30.00"},
+                {"category_id": sweets, "amount": "30.00"},
             ],
         ),
     )
@@ -623,5 +671,5 @@ async def test_delete_transaction_cascades_its_splits(client: AsyncClient, accou
 
     # 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})
+    resp = await client.get("/transactions", params={"category_id": sweets})
     assert resp.json()["total"] == 0

frontend/src/components/dashboard/SpendingByCategoryCard.tsx

@@ -1,10 +1,12 @@
+import { useState } from "react";
+import { ChevronDown, ChevronRight } from "lucide-react";
 import { Cell, Pie, PieChart, ResponsiveContainer, Tooltip } from "recharts";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/Card";
 import { getCategoryIcon } from "@/lib/icons";
 import { formatCurrency } from "@/lib/format";
-import { useTranslation } from "@/lib/i18n";
+import { useTranslation, type TranslationKey } from "@/lib/i18n";
 import { translateCategoryName } from "@/lib/categoryLabels";
-import type { CategoryBreakdownItem } from "@/types";
+import type { CategoryBreakdownChildItem, CategoryBreakdownItem } from "@/types";
 
 interface SpendingByCategoryCardProps {
   items: CategoryBreakdownItem[];
@@ -23,13 +25,31 @@ function DonutTooltip({ active, payload }: { active?: boolean; payload?: Array<{
   );
 }
 
+// A child's category_id equals its parent's own — the enclosing
+// CategoryBreakdownItem's — when that slice of spend was filed directly on
+// the parent with no more specific subcategory chosen (see
+// services/category_rollup.py's CategoryRollupChildItem).
+function childLabel(child: CategoryBreakdownChildItem, parentId: number | null, t: (key: TranslationKey) => string) {
+  return child.category_id === parentId ? t("reports.directSpendLabel") : translateCategoryName(child.name);
+}
+
 export function SpendingByCategoryCard({ items }: SpendingByCategoryCardProps) {
   const { t } = useTranslation();
+  const [expanded, setExpanded] = useState<Set<number>>(new Set());
   const hasData = items.length > 0;
   // Recharts needs a numeric dataKey — API amounts arrive as strings (Decimal
   // is serialized as string to avoid float precision loss).
   const chartData = items.map((item) => ({ ...item, amount: Number(item.amount) }));
 
+  function toggle(categoryId: number) {
+    setExpanded((prev) => {
+      const next = new Set(prev);
+      if (next.has(categoryId)) next.delete(categoryId);
+      else next.add(categoryId);
+      return next;
+    });
+  }
+
   return (
     <Card>
       <CardHeader>
@@ -66,23 +86,48 @@ export function SpendingByCategoryCard({ items }: SpendingByCategoryCardProps) {
             <ul className="w-full min-w-0 flex-1 divide-y divide-gridline">
               {items.map((item) => {
                 const Icon = getCategoryIcon(item.icon);
+                const hasChildren = item.children.length > 0;
+                const isExpanded = item.category_id !== null && expanded.has(item.category_id);
                 return (
-                  <li
-                    key={item.category_id ?? "other"}
-                    className="flex items-center gap-3 py-2 first:pt-0 last:pb-0"
-                  >
-                    <span
-                      className="flex h-7 w-7 shrink-0 items-center justify-center rounded-full"
-                      style={{ backgroundColor: `${item.color}26` }}
+                  <li key={item.category_id ?? "other"} className="py-2 first:pt-0 last:pb-0">
+                    <button
+                      type="button"
+                      disabled={!hasChildren}
+                      onClick={() => item.category_id !== null && toggle(item.category_id)}
+                      className="flex w-full items-center gap-3 text-left disabled:cursor-default"
                     >
-                      <Icon size={14} style={{ color: item.color }} />
-                    </span>
-                    <span className="min-w-0 flex-1 truncate text-sm text-text-primary">
-                      {translateCategoryName(item.name)}
-                    </span>
-                    <span className="shrink-0 text-sm font-medium tabular-nums text-text-primary">
-                      {formatCurrency(item.amount)}
-                    </span>
+                      <span className="flex h-4 w-4 shrink-0 items-center justify-center text-text-muted">
+                        {hasChildren ? isExpanded ? <ChevronDown size={14} /> : <ChevronRight size={14} /> : null}
+                      </span>
+                      <span
+                        className="flex h-7 w-7 shrink-0 items-center justify-center rounded-full"
+                        style={{ backgroundColor: `${item.color}26` }}
+                      >
+                        <Icon size={14} style={{ color: item.color }} />
+                      </span>
+                      <span className="min-w-0 flex-1 truncate text-sm text-text-primary">
+                        {translateCategoryName(item.name)}
+                      </span>
+                      <span className="shrink-0 text-sm font-medium tabular-nums text-text-primary">
+                        {formatCurrency(item.amount)}
+                      </span>
+                    </button>
+
+                    {hasChildren && isExpanded && (
+                      <ul className="ml-11 mt-1 space-y-1 border-l border-gridline pl-3">
+                        {item.children.map((child) => (
+                          <li key={child.category_id} className="flex items-center gap-2 text-xs">
+                            <span className="h-2 w-2 shrink-0 rounded-full" style={{ backgroundColor: child.color }} />
+                            <span className="min-w-0 flex-1 truncate text-text-secondary">
+                              {childLabel(child, item.category_id, t)}
+                            </span>
+                            <span className="shrink-0 tabular-nums text-text-secondary">
+                              {formatCurrency(child.amount)}
+                            </span>
+                          </li>
+                        ))}
+                      </ul>
+                    )}
                   </li>
                 );
               })}

frontend/src/components/reports/CategoryRankingCard.tsx

@@ -1,9 +1,11 @@
+import { useState } from "react";
+import { ChevronDown, ChevronRight } from "lucide-react";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/Card";
 import { getCategoryIcon } from "@/lib/icons";
 import { formatCurrency } from "@/lib/format";
-import { useTranslation } from "@/lib/i18n";
+import { useTranslation, type TranslationKey } from "@/lib/i18n";
 import { translateCategoryName } from "@/lib/categoryLabels";
-import type { CategoryRankingItem } from "@/types";
+import type { CategoryRankingChildItem, CategoryRankingItem } from "@/types";
 
 interface CategoryRankingCardProps {
   items: CategoryRankingItem[];
@@ -17,8 +19,26 @@ interface CategoryRankingCardProps {
  * detail chart below (locked to one category), this is "which category
  * costs the most" across the whole range at once. Clicking a row drills
  * into that category in the detail chart/transaction list below. */
+// A child's category_id equals its parent's own — the enclosing
+// CategoryRankingItem's — when that slice of spend was filed directly on
+// the parent with no more specific subcategory chosen (see
+// services/category_rollup.py's CategoryRollupChildItem).
+function childLabel(child: CategoryRankingChildItem, parentId: number, t: (key: TranslationKey) => string) {
+  return child.category_id === parentId ? t("reports.directSpendLabel") : translateCategoryName(child.name);
+}
+
 export function CategoryRankingCard({ items, isLoading, selectedCategoryId, onSelectCategory }: CategoryRankingCardProps) {
   const { t } = useTranslation();
+  const [expanded, setExpanded] = useState<Set<number>>(new Set());
+
+  function toggle(categoryId: number) {
+    setExpanded((prev) => {
+      const next = new Set(prev);
+      if (next.has(categoryId)) next.delete(categoryId);
+      else next.add(categoryId);
+      return next;
+    });
+  }
 
   return (
     <Card>
@@ -35,40 +55,72 @@ export function CategoryRankingCard({ items, isLoading, selectedCategoryId, onSe
             {items.map((item, index) => {
               const Icon = getCategoryIcon(item.icon);
               const isSelected = item.category_id === selectedCategoryId;
+              const hasChildren = item.children.length > 0;
+              const isExpanded = expanded.has(item.category_id);
               return (
                 <li key={item.category_id}>
-                  <button
-                    type="button"
-                    onClick={() => onSelectCategory(item.category_id)}
-                    className={`flex w-full items-center gap-3 rounded-lg py-2.5 text-left transition-colors hover:bg-surface-2 ${
+                  <div
+                    className={`flex items-center gap-1 rounded-lg transition-colors hover:bg-surface-2 ${
                       isSelected ? "bg-surface-2" : ""
                     }`}
                   >
-                    <span className="w-4 shrink-0 text-right text-xs tabular-nums text-text-muted">{index + 1}</span>
-                    <span
-                      className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full"
-                      style={{ backgroundColor: `${item.color}26` }}
+                    <button
+                      type="button"
+                      onClick={() => onSelectCategory(item.category_id)}
+                      className="flex flex-1 items-center gap-3 py-2.5 text-left"
                     >
-                      <Icon size={15} style={{ color: item.color }} />
-                    </span>
-                    <span className="min-w-0 flex-1 truncate text-sm text-text-primary">
-                      {translateCategoryName(item.name)}
-                    </span>
-                    <span className="hidden w-24 shrink-0 items-center gap-2 sm:flex">
-                      <span className="h-1.5 flex-1 overflow-hidden rounded-full bg-surface-2">
-                        <span
-                          className="block h-full rounded-full"
-                          style={{ width: `${item.percent}%`, backgroundColor: item.color }}
-                        />
+                      <span className="w-4 shrink-0 text-right text-xs tabular-nums text-text-muted">{index + 1}</span>
+                      <span
+                        className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full"
+                        style={{ backgroundColor: `${item.color}26` }}
+                      >
+                        <Icon size={15} style={{ color: item.color }} />
                       </span>
-                      <span className="w-9 shrink-0 text-right text-xs text-text-muted tabular-nums">
-                        {item.percent.toFixed(0)}%
+                      <span className="min-w-0 flex-1 truncate text-sm text-text-primary">
+                        {translateCategoryName(item.name)}
                       </span>
-                    </span>
-                    <span className="shrink-0 pr-1 text-sm font-medium tabular-nums text-text-primary">
-                      {formatCurrency(item.amount)}
-                    </span>
-                  </button>
+                      <span className="hidden w-24 shrink-0 items-center gap-2 sm:flex">
+                        <span className="h-1.5 flex-1 overflow-hidden rounded-full bg-surface-2">
+                          <span
+                            className="block h-full rounded-full"
+                            style={{ width: `${item.percent}%`, backgroundColor: item.color }}
+                          />
+                        </span>
+                        <span className="w-9 shrink-0 text-right text-xs text-text-muted tabular-nums">
+                          {item.percent.toFixed(0)}%
+                        </span>
+                      </span>
+                      <span className="shrink-0 text-sm font-medium tabular-nums text-text-primary">
+                        {formatCurrency(item.amount)}
+                      </span>
+                    </button>
+                    {hasChildren && (
+                      <button
+                        type="button"
+                        aria-label={isExpanded ? t("common.collapse") : t("common.expand")}
+                        onClick={() => toggle(item.category_id)}
+                        className="mr-1 shrink-0 rounded-md p-1.5 text-text-muted hover:text-text-primary"
+                      >
+                        {isExpanded ? <ChevronDown size={14} /> : <ChevronRight size={14} />}
+                      </button>
+                    )}
+                  </div>
+
+                  {hasChildren && isExpanded && (
+                    <ul className="ml-11 mb-2 mt-1 space-y-1 border-l border-gridline pl-3">
+                      {item.children.map((child) => (
+                        <li key={child.category_id} className="flex items-center gap-2 text-xs">
+                          <span className="h-2 w-2 shrink-0 rounded-full" style={{ backgroundColor: child.color }} />
+                          <span className="min-w-0 flex-1 truncate text-text-secondary">
+                            {childLabel(child, item.category_id, t)}
+                          </span>
+                          <span className="shrink-0 tabular-nums text-text-secondary">
+                            {formatCurrency(child.amount)}
+                          </span>
+                        </li>
+                      ))}
+                    </ul>
+                  )}
                 </li>
               );
             })}

frontend/src/components/transactions/TransactionFormModal.tsx

@@ -69,10 +69,23 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
   useEffect(() => {
     if (!open) return;
     if (transaction) {
+      const hasSplits = transaction.splits.length > 0;
+      // A split's categories always share one parent (see
+      // routes/transactions.py's _build_splits) — derive that shared base
+      // from whichever split's category is still live. If every split's
+      // category was since deleted, there's nothing to derive from; the
+      // base field is left blank and the user has to pick one again.
+      const baseCategory = hasSplits ? transaction.splits.find((split) => split.category)?.category : null;
       setForm({
         type: transaction.type,
         account_id: String(transaction.account_id),
-        category_id: transaction.category_id ? String(transaction.category_id) : "",
+        category_id: hasSplits
+          ? baseCategory
+            ? String(baseCategory.parent_id ?? baseCategory.id)
+            : ""
+          : transaction.category_id
+            ? String(transaction.category_id)
+            : "",
         transfer_account_id: transaction.transfer_account_id ? String(transaction.transfer_account_id) : "",
         amount: transaction.amount,
         description: transaction.description,
@@ -81,7 +94,6 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
         date: transaction.date,
       });
       setTags(transaction.tags);
-      const hasSplits = transaction.splits.length > 0;
       setSplitMode(hasSplits);
       setSplitRows(
         hasSplits
@@ -134,6 +146,34 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
   const splitAllocatedCents = splitRows.reduce((sum, row) => sum + toCents(row.amount), 0);
   const splitRemainingCents = toCents(form.amount) - splitAllocatedCents;
 
+  // The category select becomes the split's "base" category while
+  // splitting — restricted to top-level categories — and each split row can
+  // only pick that base itself or one of its direct children (see
+  // routes/transactions.py's _build_splits: a split's categories always
+  // share one parent).
+  const topLevelCategories = relevantCategories.filter((category) => !category.indented);
+  const baseChildCategories = kindCategories.filter((category) => category.parent_id === Number(form.category_id));
+  const categorySelectOptions = splitMode ? topLevelCategories : relevantCategories;
+
+  function toggleSplitMode() {
+    setSplitMode((prev) => {
+      const next = !prev;
+      if (next) {
+        const current = relevantCategories.find((category) => String(category.id) === form.category_id);
+        if (current?.parent_id) {
+          setForm((f) => ({ ...f, category_id: String(current.parent_id) }));
+        }
+        setSplitRows([emptySplitRow(), emptySplitRow()]);
+      }
+      return next;
+    });
+  }
+
+  function handleBaseCategoryChange(value: string) {
+    setForm((prev) => ({ ...prev, category_id: value }));
+    if (splitMode) setSplitRows([emptySplitRow(), emptySplitRow()]);
+  }
+
   async function handleSubmit(event: React.FormEvent) {
     event.preventDefault();
     setError(null);
@@ -157,6 +197,10 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
     // switched the transaction to a transfer, which can't carry splits).
     let splits: TransactionSplitInput[] | undefined;
     if (isSplitEditingNow) {
+      if (!form.category_id) {
+        setError(t("transactions.form.errorSplitNoBaseCategory"));
+        return;
+      }
       const filledRows = splitRows.filter((row) => row.category_id || row.amount);
       if (filledRows.length < 2) {
         setError(t("transactions.form.errorSplitMinRows"));
@@ -318,29 +362,35 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
               <button
                 type="button"
                 className="mb-1 text-xs text-series-1 hover:underline"
-                onClick={() => setSplitMode((prev) => !prev)}
+                onClick={toggleSplitMode}
               >
                 {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>
+            <Select
+              id="category"
+              value={form.category_id}
+              onChange={(event) => handleBaseCategoryChange(event.target.value)}
+            >
+              <option value="">{t("transactions.form.noCategory")}</option>
+              {categorySelectOptions.map((category) => (
+                <option key={category.id} value={category.id}>
+                  {category.indented ? `    ↳ ` : ""}
+                  {translateCategoryName(category.name)}
+                </option>
+              ))}
+            </Select>
+
+            {splitMode && (
+              <div className="mt-2 space-y-2">
+                {!form.category_id ? (
+                  <p className="text-xs text-text-muted">{t("transactions.form.splitHint")}</p>
+                ) : baseChildCategories.length === 0 ? (
+                  <p className="text-xs text-text-muted">{t("transactions.form.splitNoChildren")}</p>
+                ) : (
+                  <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">
@@ -348,14 +398,17 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
                         aria-label={t("transactions.form.splitCategoryPlaceholder")}
                         className="sm:flex-1"
                         value={row.category_id}
+                        disabled={!form.category_id}
                         onChange={(event) => updateSplitRow(row.key, { category_id: event.target.value })}
                       >
                         <option value="" disabled>
                           {t("transactions.form.splitCategoryPlaceholder")}
                         </option>
-                        {relevantCategories.map((category) => (
+                        {form.category_id && (
+                          <option value={form.category_id}>{t("transactions.form.splitDirectOption")}</option>
+                        )}
+                        {baseChildCategories.map((category) => (
                           <option key={category.id} value={category.id}>
-                            {category.indented ? `    ↳ ` : ""}
                             {translateCategoryName(category.name)}
                           </option>
                         ))}

frontend/src/lib/i18n.ts

@@ -29,6 +29,8 @@ const ru = {
   "topbar.openMenu": "Открыть меню",
 
   "common.loading": "Загрузка…",
+  "common.expand": "Развернуть",
+  "common.collapse": "Свернуть",
   "common.back": "Назад",
   "common.next": "Далее",
   "common.clear": "Очистить",
@@ -285,6 +287,10 @@ const ru = {
   "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",
+  "transactions.form.errorSplitNoBaseCategory": "Сначала выберите базовую категорию",
+  "transactions.form.splitDirectOption": "— без конкретной подкатегории —",
+  "transactions.form.splitNoChildren": "У этой категории пока нет подкатегорий — добавьте их на странице «Категории», чтобы разбивать покупки по ним.",
+  "reports.directSpendLabel": "Без подкатегории",
 
   "reports.categoryLabel": "Категория",
   "reports.expenseGroup": "Расходы",
@@ -504,6 +510,8 @@ const en: Record<keyof typeof ru, string> = {
   "topbar.openMenu": "Open menu",
 
   "common.loading": "Loading…",
+  "common.expand": "Expand",
+  "common.collapse": "Collapse",
   "common.back": "Back",
   "common.next": "Next",
   "common.clear": "Clear",
@@ -760,6 +768,10 @@ const en: Record<keyof typeof ru, string> = {
   "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",
+  "transactions.form.errorSplitNoBaseCategory": "Pick a base category first",
+  "transactions.form.splitDirectOption": "— no specific subcategory —",
+  "transactions.form.splitNoChildren": "This category has no subcategories yet — add some on the Categories page to split purchases across them.",
+  "reports.directSpendLabel": "No subcategory",
 
   "reports.categoryLabel": "Category",
   "reports.expenseGroup": "Expenses",

frontend/src/types/index.ts

@@ -161,6 +161,18 @@ export interface RecurringTransactionInput {
   anchor_date: string;
 }
 
+// One subcategory's (or the parent's own direct, un-subcategorized) share
+// of a CategoryBreakdownItem's total. Populated only when more than one
+// distinct category fed the slice — a single-source category (the common
+// case) leaves this off the parent item entirely.
+export interface CategoryBreakdownChildItem {
+  category_id: number;
+  name: string;
+  color: string;
+  icon: string | null;
+  amount: string;
+}
+
 export interface CategoryBreakdownItem {
   category_id: number | null;
   name: string;
@@ -168,6 +180,7 @@ export interface CategoryBreakdownItem {
   icon: string | null;
   amount: string;
   percent: number;
+  children: CategoryBreakdownChildItem[];
 }
 
 export interface DashboardSummary {
@@ -310,6 +323,14 @@ export interface CashFlowResponse {
   total_net: string;
 }
 
+export interface CategoryRankingChildItem {
+  category_id: number;
+  name: string;
+  color: string;
+  icon: string | null;
+  amount: string;
+}
+
 export interface CategoryRankingItem {
   category_id: number;
   name: string;
@@ -318,6 +339,7 @@ export interface CategoryRankingItem {
   amount: string;
   percent: number;
   transaction_count: number;
+  children: CategoryRankingChildItem[];
 }
 
 export interface CategoryRankingReport {