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/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