fix: snapshot nullable columns correctly in WatchedFieldsMixin callback (#194)

src/fastapi_toolsets/models/watched.py

@@ -27,6 +27,7 @@ _SESSION_CREATES = "_ft_creates"
 _SESSION_DELETES = "_ft_deletes"
 _SESSION_UPDATES = "_ft_updates"
 _SESSION_SAVEPOINT_DEPTH = "_ft_sp_depth"
+_DEFERRED_STRATEGY_KEY = (("deferred", True), ("instrument", True))
 
 
 class ModelEvent(str, Enum):
@@ -60,11 +61,22 @@ def _snapshot_column_attrs(obj: Any) -> dict[str, Any]:
     """Read currently-loaded column values into a plain dict."""
     state = sa_inspect(obj)  # InstanceState
     state_dict = state.dict
-    return {
+    snapshot: dict[str, Any] = {}
-        prop.key: state_dict[prop.key]
+    for prop in state.mapper.column_attrs:
-        for prop in state.mapper.column_attrs
+        if prop.key in state_dict:
-        if prop.key in state_dict
+            snapshot[prop.key] = state_dict[prop.key]
-    }
+        elif (
+            not state.expired
+            and prop.strategy_key != _DEFERRED_STRATEGY_KEY
+            and all(
+                col.nullable
+                and col.server_default is None
+                and col.server_onupdate is None
+                for col in prop.columns
+            )
+        ):
+            snapshot[prop.key] = None
+    return snapshot
 
 
 def _get_watched_fields(cls: type) -> list[str] | None:

tests/test_models.py

@@ -32,6 +32,7 @@ from fastapi_toolsets.models.watched import (
     _after_flush,
     _after_flush_postexec,
     _after_rollback,
+    _snapshot_column_attrs,
     _task_error_handler,
     _upsert_changes,
 )
@@ -213,20 +214,36 @@ class AttrAccessModel(MixinBase, UUIDMixin, WatchedFieldsMixin):
     __tablename__ = "mixin_attr_access_models"
 
     name: Mapped[str] = mapped_column(String(50))
+    callback_url: Mapped[str | None] = mapped_column(String(200), nullable=True)
 
     async def on_create(self) -> None:
         _attr_access_events.append(
-            {"event": "create", "id": self.id, "name": self.name}
+            {
+                "event": "create",
+                "id": self.id,
+                "name": self.name,
+                "callback_url": self.callback_url,
+            }
         )
 
     async def on_delete(self) -> None:
         _attr_access_events.append(
-            {"event": "delete", "id": self.id, "name": self.name}
+            {
+                "event": "delete",
+                "id": self.id,
+                "name": self.name,
+                "callback_url": self.callback_url,
+            }
         )
 
     async def on_update(self, changes: dict) -> None:
         _attr_access_events.append(
-            {"event": "update", "id": self.id, "name": self.name}
+            {
+                "event": "update",
+                "id": self.id,
+                "name": self.name,
+                "callback_url": self.callback_url,
+            }
         )
 
 
@@ -1279,3 +1296,67 @@ class TestAttributeAccessInCallbacks:
         assert len(events) == 1
         assert isinstance(events[0]["id"], uuid.UUID)
         assert events[0]["name"] == "updated"
+
+    @pytest.mark.anyio
+    async def test_nullable_column_none_accessible_in_on_create(
+        self, mixin_session_expire
+    ):
+        """Nullable column left as None is accessible in on_create without greenlet error."""
+        obj = AttrAccessModel(name="no-url")  # callback_url not set → None
+        mixin_session_expire.add(obj)
+        await mixin_session_expire.commit()
+        await asyncio.sleep(0)
+
+        events = [e for e in _attr_access_events if e["event"] == "create"]
+        assert len(events) == 1
+        assert events[0]["callback_url"] is None
+
+    @pytest.mark.anyio
+    async def test_nullable_column_with_value_accessible_in_on_create(
+        self, mixin_session_expire
+    ):
+        """Nullable column set to a value is accessible in on_create without greenlet error."""
+        obj = AttrAccessModel(name="with-url", callback_url="https://example.com/hook")
+        mixin_session_expire.add(obj)
+        await mixin_session_expire.commit()
+        await asyncio.sleep(0)
+
+        events = [e for e in _attr_access_events if e["event"] == "create"]
+        assert len(events) == 1
+        assert events[0]["callback_url"] == "https://example.com/hook"
+
+    @pytest.mark.anyio
+    async def test_nullable_column_accessible_after_update_to_none(
+        self, mixin_session_expire
+    ):
+        """Nullable column updated to None is accessible in on_update without greenlet error."""
+        obj = AttrAccessModel(name="x", callback_url="https://example.com/hook")
+        mixin_session_expire.add(obj)
+        await mixin_session_expire.commit()
+        await asyncio.sleep(0)
+        _attr_access_events.clear()
+
+        obj.callback_url = None
+        await mixin_session_expire.commit()
+        await asyncio.sleep(0)
+
+        events = [e for e in _attr_access_events if e["event"] == "update"]
+        assert len(events) == 1
+        assert events[0]["callback_url"] is None
+
+    @pytest.mark.anyio
+    async def test_expired_nullable_column_not_inferred_as_none(
+        self, mixin_session_expire
+    ):
+        """A nullable column with a real value that is expired (by a prior
+        expire_on_commit) must not be inferred as None in the snapshot — its
+        actual value is unknown without a DB refresh."""
+        obj = AttrAccessModel(name="original", callback_url="https://example.com/hook")
+        mixin_session_expire.add(obj)
+        await mixin_session_expire.commit()
+        # expire_on_commit fired → obj.state.expired=True, callback_url not in state.dict
+
+        snapshot = _snapshot_column_attrs(obj)
+
+        # callback_url has a real DB value but is expired — must not be snapshotted as None.
+        assert "callback_url" not in snapshot