fix(budget): cascade-delete budget lines when deleting a budget

services/budget/app/models/budget.py

@@ -77,7 +77,10 @@ class BudgetModel(Base, AuditMixin):
     excel_import_structure: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     excel_import_lines_locked_count: Mapped[int | None] = mapped_column(Integer, nullable=True)
     lines: Mapped[list["BudgetLineModel"]] = relationship(
-        "BudgetLineModel", back_populates="budget"
+        "BudgetLineModel",
+        back_populates="budget",
+        cascade="all, delete-orphan",
+        passive_deletes=True,
     )
     reports: Mapped[list["ReportModel"]] = relationship("ReportModel", back_populates="budget")
     categories: Mapped[list["BudgetCategoryModel"]] = relationship(
@@ -94,7 +97,9 @@ class BudgetLineModel(Base, AuditMixin):
     id: Mapped[uuid.UUID] = mapped_column(
         GUID(), primary_key=True, index=True, default=lambda: uuid.uuid4()
     )
-    budget_id: Mapped[uuid.UUID] = mapped_column(GUID(), ForeignKey("budgets.id"), nullable=False)
+    budget_id: Mapped[uuid.UUID] = mapped_column(
+        GUID(), ForeignKey("budgets.id", ondelete="CASCADE"), nullable=False
+    )
     category_id: Mapped[uuid.UUID | None] = mapped_column(
         GUID(), ForeignKey("budget_categories.id", ondelete="SET NULL"), nullable=True
     )

services/budget/migrations/versions/000021_budget_lines_budget_id_cascade.py

@@ -0,0 +1,35 @@
+"""Cascade-delete budget lines with their budget
+
+Revision ID: 000021
+Revises: 000020
+Create Date: 2026-10-01 00:00:00.000000
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "000021"
+down_revision: Union[str, Sequence[str], None] = "000020"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_constraint("budget_lines_budget_id_fkey", "budget_lines", type_="foreignkey")
+    op.create_foreign_key(
+        "budget_lines_budget_id_fkey",
+        "budget_lines",
+        "budgets",
+        ["budget_id"],
+        ["id"],
+        ondelete="CASCADE",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("budget_lines_budget_id_fkey", "budget_lines", type_="foreignkey")
+    op.create_foreign_key(
+        "budget_lines_budget_id_fkey", "budget_lines", "budgets", ["budget_id"], ["id"]
+    )

services/budget/tests/test_budget_crud.py

@@ -117,3 +117,44 @@ async def test_delete_succeeds_with_categories_already_loaded(self, db):
         assert len(list(loaded_budget.categories)) == 1
 
         assert await delete_budget(session=db, budget=loaded_budget) is True
+
+
+async def _budget_with_line(db) -> BudgetModel:
+    user = ValidUserFactory()
+    budget = await create_budget(
+        session=db, user_id=user["user_id"], name="WithLines", owner_id=user["customer_id"]
+    )
+    category = await create_budget_category(
+        session=db, user_id=user["user_id"], budget_id=budget.id, name="Travel"
+    )
+    await create_budget_line(
+        session=db,
+        user_id=user["user_id"],
+        budget_id=budget.id,
+        category_id=category.id,
+        description="Flight",
+        amount=100.0,
+    )
+    return budget
+
+
+@pytest.mark.anyio
+class TestDeleteBudgetCascadesLines:
+    async def test_delete_succeeds_with_lines_unloaded(self, db):
+        budget = await _budget_with_line(db)
+
+        # sqlite doesn't enforce the DB-level cascade; Postgres removes the lines.
+        assert await delete_budget(session=db, budget=budget) is True
+
+    async def test_delete_succeeds_with_lines_already_loaded(self, db):
+        budget = await _budget_with_line(db)
+
+        result = await db.execute(
+            select(BudgetModel)
+            .where(BudgetModel.id == budget.id)
+            .options(selectinload(BudgetModel.lines))
+        )
+        loaded_budget = result.scalar_one()
+        assert len(list(loaded_budget.lines)) == 1
+
+        assert await delete_budget(session=db, budget=loaded_budget) is True