Remove models.Model inheritance from ChangesMixin

ChangesMixin is now a plain Python mixin class that works when combined
with models.Model in user code. This fixes AppRegistryNotReady errors
when packages that import ChangesMixin are in INSTALLED_APPS.

- ChangesMixin no longer inherits from models.Model
- Signals connect once per class on first instantiation
- Users must now explicitly inherit from both: class MyModel(ChangesMixin, models.Model)

Author: Test User

django_model_changes/changes.py

@@ -13,11 +13,16 @@
 SAVE = 0
 DELETE = 1
 
+# Track which classes have had signals connected
+_connected_classes: set[type] = set()
 
-class ChangesMixin(models.Model):
+
+class ChangesMixin:
     r"""
     ChangesMixin keeps track of changes for model instances.
 
+    Use with models.Model: ``class MyModel(ChangesMixin, models.Model)``
+
     It allows you to retrieve the following states from an instance:
 
     1. current_state()
@@ -70,27 +75,33 @@ class ChangesMixin(models.Model):
     """
 
     _states: list[dict[str, Any]]
-
-    class Meta:
-        abstract = True
+    _meta: Options
+    pk: Any
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
         super().__init__(*args, **kwargs)
 
+        # Connect signals once per class (on first instantiation)
+        cls = self.__class__
+        if cls not in _connected_classes:
+            _connected_classes.add(cls)
+            # cls is guaranteed to be a Model subclass at runtime since ChangesMixin
+            # must be used with models.Model
+            sender: type[models.Model] = cls  # type: ignore[assignment]
+            signals.post_save.connect(
+                _post_save,
+                sender=sender,
+                dispatch_uid=f"django-changes-{cls.__name__}",
+            )
+            signals.post_delete.connect(
+                _post_delete,
+                sender=sender,
+                dispatch_uid=f"django-changes-{cls.__name__}",
+            )
+
         self._states = []
         self._save_state(new_instance=True)
 
-        signals.post_save.connect(
-            _post_save,
-            sender=self.__class__,
-            dispatch_uid=f"django-changes-{self.__class__.__name__}",
-        )
-        signals.post_delete.connect(
-            _post_delete,
-            sender=self.__class__,
-            dispatch_uid=f"django-changes-{self.__class__.__name__}",
-        )
-
     def _save_state(self, new_instance: bool = False, event_type: str | int = "save") -> None:
         # Pipe the pk on deletes so that a correct snapshot of the current
         # state can be taken.

tests/models.py

@@ -1,13 +1,13 @@
 from django.db import models
 
-from django_model_changes.changes import ChangesMixin
+from django_model_changes import ChangesMixin
 
 
-class User(ChangesMixin):
+class User(ChangesMixin, models.Model):
     name = models.CharField(max_length=100)
     flag = models.BooleanField(default=False)
 
 
-class Article(ChangesMixin):
+class Article(ChangesMixin, models.Model):
     title = models.CharField(max_length=20)
     user = models.ForeignKey(User, on_delete=models.CASCADE)