feat: add wait_for_row_change db helper (#49)

tests/test_db.py

@@ -1,5 +1,8 @@
 """Tests for fastapi_toolsets.db module."""
 
+import asyncio
+import uuid
+
 import pytest
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 
@@ -9,6 +12,7 @@ from fastapi_toolsets.db import (
     create_db_dependency,
     get_transaction,
     lock_tables,
+    wait_for_row_change,
 )
 
 from .conftest import DATABASE_URL, Base, Role, RoleCrud, User
@@ -241,3 +245,101 @@ class TestLockTables:
 
         result = await RoleCrud.first(db_session, [Role.name == "lock_rollback_role"])
         assert result is None
+
+
+class TestWaitForRowChange:
+    """Tests for wait_for_row_change polling function."""
+
+    @pytest.mark.anyio
+    async def test_detects_update(self, db_session: AsyncSession, engine):
+        """Returns updated instance when a column value changes."""
+        role = Role(name="watch_role")
+        db_session.add(role)
+        await db_session.commit()
+
+        async def update_later():
+            await asyncio.sleep(0.15)
+            factory = async_sessionmaker(engine, expire_on_commit=False)
+            async with factory() as other:
+                r = await other.get(Role, role.id)
+                assert r is not None
+                r.name = "updated_role"
+                await other.commit()
+
+        update_task = asyncio.create_task(update_later())
+        result = await wait_for_row_change(db_session, Role, role.id, interval=0.05)
+        await update_task
+
+        assert result.name == "updated_role"
+
+    @pytest.mark.anyio
+    async def test_watches_specific_columns(self, db_session: AsyncSession, engine):
+        """Only triggers on changes to specified columns."""
+        user = User(username="testuser", email="test@example.com")
+        db_session.add(user)
+        await db_session.commit()
+
+        async def update_later():
+            factory = async_sessionmaker(engine, expire_on_commit=False)
+            # First: change email (not watched) — should not trigger
+            await asyncio.sleep(0.15)
+            async with factory() as other:
+                u = await other.get(User, user.id)
+                assert u is not None
+                u.email = "new@example.com"
+                await other.commit()
+            # Second: change username (watched) — should trigger
+            await asyncio.sleep(0.15)
+            async with factory() as other:
+                u = await other.get(User, user.id)
+                assert u is not None
+                u.username = "newuser"
+                await other.commit()
+
+        update_task = asyncio.create_task(update_later())
+        result = await wait_for_row_change(
+            db_session, User, user.id, columns=["username"], interval=0.05
+        )
+        await update_task
+
+        assert result.username == "newuser"
+        assert result.email == "new@example.com"
+
+    @pytest.mark.anyio
+    async def test_nonexistent_row_raises(self, db_session: AsyncSession):
+        """Raises LookupError when the row does not exist."""
+        fake_id = uuid.uuid4()
+        with pytest.raises(LookupError, match="not found"):
+            await wait_for_row_change(db_session, Role, fake_id, interval=0.05)
+
+    @pytest.mark.anyio
+    async def test_timeout_raises(self, db_session: AsyncSession):
+        """Raises TimeoutError when no change is detected within timeout."""
+        role = Role(name="timeout_role")
+        db_session.add(role)
+        await db_session.commit()
+
+        with pytest.raises(TimeoutError):
+            await wait_for_row_change(
+                db_session, Role, role.id, interval=0.05, timeout=0.2
+            )
+
+    @pytest.mark.anyio
+    async def test_deleted_row_raises(self, db_session: AsyncSession, engine):
+        """Raises LookupError when the row is deleted during polling."""
+        role = Role(name="delete_role")
+        db_session.add(role)
+        await db_session.commit()
+
+        async def delete_later():
+            await asyncio.sleep(0.15)
+            factory = async_sessionmaker(engine, expire_on_commit=False)
+            async with factory() as other:
+                r = await other.get(Role, role.id)
+                await other.delete(r)
+                await other.commit()
+
+        delete_task = asyncio.create_task(delete_later())
+        with pytest.raises(LookupError):
+            await wait_for_row_change(db_session, Role, role.id, interval=0.05)
+        await delete_task