fix: resolve MissingGreenlet error when accessing self attributes in WatchedFieldsMixin callbacks (#154)

src/fastapi_toolsets/models/watched.py

@@ -9,6 +9,7 @@ from typing import Any, TypeVar
 from sqlalchemy import event
 from sqlalchemy import inspect as sa_inspect
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm.attributes import set_committed_value as _sa_set_committed_value
 
 from ..logger import get_logger
 
@@ -53,6 +54,17 @@ def watch(*fields: str) -> Any:
     return decorator
 
 
+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 {
+        prop.key: state_dict[prop.key]
+        for prop in state.mapper.column_attrs
+        if prop.key in state_dict
+    }
+
+
 def _upsert_changes(
     pending: dict[int, tuple[Any, dict[str, dict[str, Any]]]],
     obj: Any,
@@ -139,16 +151,31 @@ def _task_error_handler(task: asyncio.Task[Any]) -> None:
         _logger.error(_CALLBACK_ERROR_MSG, exc_info=exc)
 
 
-def _call_callback(loop: asyncio.AbstractEventLoop, fn: Any, *args: Any) -> None:
-    """Dispatch *fn* with *args*, handling both sync and async callables."""
-    try:
-        result = fn(*args)
-    except Exception as exc:
-        _logger.error(_CALLBACK_ERROR_MSG, exc_info=exc)
-        return
-    if asyncio.iscoroutine(result):
-        task = loop.create_task(result)
-        task.add_done_callback(_task_error_handler)
+def _schedule_with_snapshot(
+    loop: asyncio.AbstractEventLoop, obj: Any, fn: Any, *args: Any
+) -> None:
+    """Snapshot *obj*'s column attrs now (before expire_on_commit wipes them),
+    then schedule a coroutine that restores the snapshot and calls *fn*.
+    """
+    snapshot = _snapshot_column_attrs(obj)
+
+    async def _run(
+        obj: Any = obj,
+        fn: Any = fn,
+        snapshot: dict[str, Any] = snapshot,
+        args: tuple = args,
+    ) -> None:
+        for key, value in snapshot.items():
+            _sa_set_committed_value(obj, key, value)
+        try:
+            result = fn(*args)
+            if asyncio.iscoroutine(result):
+                await result
+        except Exception as exc:
+            _logger.error(_CALLBACK_ERROR_MSG, exc_info=exc)
+
+    task = loop.create_task(_run())
+    task.add_done_callback(_task_error_handler)
 
 
 @event.listens_for(AsyncSession.sync_session_class, "after_commit")
@@ -168,13 +195,13 @@ def _after_commit(session: Any) -> None:
         return
 
     for obj in creates:
-        _call_callback(loop, obj.on_create)
+        _schedule_with_snapshot(loop, obj, obj.on_create)
 
     for obj in deletes:
-        _call_callback(loop, obj.on_delete)
+        _schedule_with_snapshot(loop, obj, obj.on_delete)
 
     for obj, changes in field_changes.values():
-        _call_callback(loop, obj.on_update, changes)
+        _schedule_with_snapshot(loop, obj, obj.on_update, changes)
 
 
 class WatchedFieldsMixin: