feat: add WatchedFieldsMixin (#148)

* feat/add WatchedFieldsMixin and watch_fields decorator for field-change monitoring

* docs: add WatchedFieldsMixin

* feat: add on_event, on_create and on_delete

* docs: update README

src/fastapi_toolsets/models/__init__.py

@@ -0,0 +1,21 @@
+"""SQLAlchemy model mixins for common column patterns."""
+
+from .columns import (
+    CreatedAtMixin,
+    TimestampMixin,
+    UUIDMixin,
+    UUIDv7Mixin,
+    UpdatedAtMixin,
+)
+from .watched import ModelEvent, WatchedFieldsMixin, watch
+
+__all__ = [
+    "ModelEvent",
+    "UUIDMixin",
+    "UUIDv7Mixin",
+    "CreatedAtMixin",
+    "UpdatedAtMixin",
+    "TimestampMixin",
+    "WatchedFieldsMixin",
+    "watch",
+]

src/fastapi_toolsets/models/columns.py

@@ -1,4 +1,4 @@
-"""SQLAlchemy model mixins for common column patterns."""
+"""SQLAlchemy column mixins for common column patterns."""
 
 import uuid
 from datetime import datetime

src/fastapi_toolsets/models/watched.py

@@ -0,0 +1,204 @@
+"""Field-change monitoring via SQLAlchemy session events."""
+
+import asyncio
+import weakref
+from collections.abc import Awaitable
+from enum import Enum
+from typing import Any, TypeVar
+
+from sqlalchemy import event
+from sqlalchemy import inspect as sa_inspect
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from ..logger import get_logger
+
+__all__ = ["ModelEvent", "WatchedFieldsMixin", "watch"]
+
+_logger = get_logger()
+_T = TypeVar("_T")
+_CALLBACK_ERROR_MSG = "WatchedFieldsMixin callback raised an unhandled exception"
+_WATCHED_FIELDS: weakref.WeakKeyDictionary[type, list[str]] = (
+    weakref.WeakKeyDictionary()
+)
+_SESSION_PENDING_NEW = "_ft_pending_new"
+_SESSION_CREATES = "_ft_creates"
+_SESSION_DELETES = "_ft_deletes"
+_SESSION_UPDATES = "_ft_updates"
+
+
+class ModelEvent(str, Enum):
+    """Event types emitted by :class:`WatchedFieldsMixin`."""
+
+    CREATE = "create"
+    DELETE = "delete"
+    UPDATE = "update"
+
+
+def watch(*fields: str) -> Any:
+    """Class decorator to filter which fields trigger ``on_update``.
+
+    Args:
+        *fields: One or more field names to watch. At least one name is required.
+
+    Raises:
+        ValueError: If called with no field names.
+    """
+    if not fields:
+        raise ValueError("@watch requires at least one field name.")
+
+    def decorator(cls: type[_T]) -> type[_T]:
+        _WATCHED_FIELDS[cls] = list(fields)
+        return cls
+
+    return decorator
+
+
+def _upsert_changes(
+    pending: dict[int, tuple[Any, dict[str, dict[str, Any]]]],
+    obj: Any,
+    changes: dict[str, dict[str, Any]],
+) -> None:
+    """Insert or merge *changes* into *pending* for *obj*."""
+    key = id(obj)
+    if key in pending:
+        existing = pending[key][1]
+        for field, change in changes.items():
+            if field in existing:
+                existing[field]["new"] = change["new"]
+            else:
+                existing[field] = change
+    else:
+        pending[key] = (obj, changes)
+
+
+@event.listens_for(AsyncSession.sync_session_class, "after_flush")
+def _after_flush(session: Any, flush_context: Any) -> None:
+    # New objects: capture references while session.new is still populated.
+    # Values are read in _after_flush_postexec once RETURNING has been processed.
+    for obj in session.new:
+        if isinstance(obj, WatchedFieldsMixin):
+            session.info.setdefault(_SESSION_PENDING_NEW, []).append(obj)
+
+    # Deleted objects: capture before they leave the identity map.
+    for obj in session.deleted:
+        if isinstance(obj, WatchedFieldsMixin):
+            session.info.setdefault(_SESSION_DELETES, []).append(obj)
+
+    # Dirty objects: read old/new from SQLAlchemy attribute history.
+    for obj in session.dirty:
+        if not isinstance(obj, WatchedFieldsMixin):
+            continue
+
+        # None = not in dict = watch all fields; list = specific fields only
+        watched = _WATCHED_FIELDS.get(type(obj))
+        changes: dict[str, dict[str, Any]] = {}
+
+        attrs = (
+            # Specific fields
+            ((field, sa_inspect(obj).attrs[field]) for field in watched)
+            if watched is not None
+            # All mapped fields
+            else ((s.key, s) for s in sa_inspect(obj).attrs)
+        )
+        for field, attr_state in attrs:
+            history = attr_state.history
+            if history.has_changes() and history.deleted:
+                changes[field] = {
+                    "old": history.deleted[0],
+                    "new": history.added[0] if history.added else None,
+                }
+
+        if changes:
+            _upsert_changes(
+                session.info.setdefault(_SESSION_UPDATES, {}),
+                obj,
+                changes,
+            )
+
+
+@event.listens_for(AsyncSession.sync_session_class, "after_flush_postexec")
+def _after_flush_postexec(session: Any, flush_context: Any) -> None:
+    # New objects are now persistent and RETURNING values have been applied,
+    # so server defaults (id, created_at, …) are available via getattr.
+    pending_new: list[Any] = session.info.pop(_SESSION_PENDING_NEW, [])
+    if not pending_new:
+        return
+    session.info.setdefault(_SESSION_CREATES, []).extend(pending_new)
+
+
+@event.listens_for(AsyncSession.sync_session_class, "after_rollback")
+def _after_rollback(session: Any) -> None:
+    session.info.pop(_SESSION_PENDING_NEW, None)
+    session.info.pop(_SESSION_CREATES, None)
+    session.info.pop(_SESSION_DELETES, None)
+    session.info.pop(_SESSION_UPDATES, None)
+
+
+def _task_error_handler(task: asyncio.Task[Any]) -> None:
+    if not task.cancelled() and (exc := task.exception()):
+        _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)
+
+
+@event.listens_for(AsyncSession.sync_session_class, "after_commit")
+def _after_commit(session: Any) -> None:
+    creates: list[Any] = session.info.pop(_SESSION_CREATES, [])
+    deletes: list[Any] = session.info.pop(_SESSION_DELETES, [])
+    field_changes: dict[int, tuple[Any, dict[str, dict[str, Any]]]] = session.info.pop(
+        _SESSION_UPDATES, {}
+    )
+
+    if not creates and not deletes and not field_changes:
+        return
+
+    try:
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        return
+
+    for obj in creates:
+        _call_callback(loop, obj.on_create)
+
+    for obj in deletes:
+        _call_callback(loop, obj.on_delete)
+
+    for obj, changes in field_changes.values():
+        _call_callback(loop, obj.on_update, changes)
+
+
+class WatchedFieldsMixin:
+    """Mixin that enables lifecycle callbacks for SQLAlchemy models."""
+
+    def on_event(
+        self, event: ModelEvent, changes: dict[str, dict[str, Any]] | None = None
+    ) -> Awaitable[None] | None:
+        """Catch-all callback fired for every lifecycle event.
+
+        Args:
+            event: The event type (:attr:`ModelEvent.CREATE`, :attr:`ModelEvent.DELETE`,
+                or :attr:`ModelEvent.UPDATE`).
+            changes: Field changes for :attr:`ModelEvent.UPDATE`, ``None`` otherwise.
+        """
+
+    def on_create(self) -> Awaitable[None] | None:
+        """Called after INSERT commit."""
+        return self.on_event(ModelEvent.CREATE)
+
+    def on_delete(self) -> Awaitable[None] | None:
+        """Called after DELETE commit."""
+        return self.on_event(ModelEvent.DELETE)
+
+    def on_update(self, changes: dict[str, dict[str, Any]]) -> Awaitable[None] | None:
+        """Called after UPDATE commit when watched fields change."""
+        return self.on_event(ModelEvent.UPDATE, changes=changes)