Improvements

README.md

@@ -72,6 +72,7 @@ Out[1]: ["I", "am", "your", "father"]
 ```
 DTM handles deferred fields well.
 ```python
+# from django.db.models.query_utils import DeferredAttribute
 In [1]: e = Example.objects.only("array").first()
 In [2]: e.text = "I am not your father" 
 In [3]: e.tracker.changed
@@ -84,6 +85,17 @@ class Example(models.Model):
     first = models.TextField()
     second = models.TextField()
 ```
+You can also implement your own Tracker class:
+```python
+from tracking_model import Tracker
+
+class SuperTracker(Tracker):
+    def has_changed(self, field):
+      return field in self.changed
+
+class Example(models.Model):
+    TRACKER_CLASS = SuperTracker
+```
 
 ## Requirements
  * Python >= 2.7, <= 3.11

tests/models.py

@@ -39,17 +39,14 @@ class NarrowTrackedModel(TrackingModelMixin, models.Model):
 
 
 class CustomTracker(Tracker):
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
     def has_changed(self, field):
         if field not in self.tracked_fields:
             raise ValueError("%s is not tracked" % field)
         return field in self.changed
 
 
 class WithCustomTrackerModel(TrackingModelMixin, models.Model):
-    tracker_class = CustomTracker
+    TRACKER_CLASS = CustomTracker
     TRACKED_FIELDS = ["first"]
     first = models.TextField(null=True)
     second = models.TextField(null=True)
@@ -60,7 +57,7 @@ class InvalidTracker:
 
 
 class WithInvalidTrackerModel(TrackingModelMixin, models.Model):
-    tracker_class = InvalidTracker
+    TRACKER_CLASS = InvalidTracker
     TRACKED_FIELDS = ["first"]
     first = models.TextField(null=True)
     second = models.TextField(null=True)

tests/test_tracking_model.py

@@ -224,11 +224,11 @@ def test_only_track_first(self):
 class OverrideTrackerTests(TestCase):
     def test_tracking_mixin_raises_error_if_tracker_class_is_invalid(self):
         with self.assertRaises(TypeError) as e:
-            WithInvalidTrackerModel(first="Joh", second="Doe")
+            WithInvalidTrackerModel(first="Joh", second="Doe").tracker
 
         self.assertEqual(
             str(e.exception),
-            "tracker_class must be subclass of Tracker.",
+            "TRACKER_CLASS must be a subclass of Tracker.",
         )
 
     def test_instance_can_use_new_methods_of_tracker_class(self):

tracking_model/mixins.py

@@ -10,37 +10,31 @@ def __init__(self, instance):
 
 
 class TrackingModelMixin(object):
-
     TRACKED_FIELDS = None
-    tracker_class = Tracker
+    # TRACKER_CLASS = Tracker
 
     def __init__(self, *args, **kwargs):
-        self._validate_tracker_class()
         super(TrackingModelMixin, self).__init__(*args, **kwargs)
         self._initialized = True
 
-    def _validate_tracker_class(self):
-        if not self.tracker_class:
-            raise AttributeError(
-                "Please set tracker_class attribute."
-            )
-
-        if not issubclass(self.tracker_class, Tracker):
-            raise TypeError(
-                "tracker_class must be subclass of Tracker."
-            )
-
     @property
     def tracker(self):
         if hasattr(self._state, "_tracker"):
             tracker = self._state._tracker
         else:
+            # add possibility to change tracker class
+            TRACKER_CLASS = getattr(self, "TRACKER_CLASS", Tracker)
+            if not issubclass(TRACKER_CLASS, Tracker):
+                raise TypeError("TRACKER_CLASS must be a subclass of Tracker.")
+
             # populate tracked fields for the first time
             # by default all fields
             if not self.TRACKED_FIELDS:
                 instance_class = type(self)
-                instance_class.TRACKED_FIELDS = {f.attname for f in instance_class._meta.concrete_fields}
-            tracker = self._state._tracker = self.tracker_class(self)
+                instance_class.TRACKED_FIELDS = {
+                    f.attname for f in instance_class._meta.concrete_fields
+                }
+            tracker = self._state._tracker = TRACKER_CLASS(self)
         return tracker
 
     def save(