fix #330 patch MyModel.save instead of MyModel().save

CHANGES.rst

@@ -6,6 +6,7 @@ master (unreleased)
 - Fix handling of deferred attributes on Django 1.10+, fixes GH-278
 - Fix `FieldTracker.has_changed()` and `FieldTracker.previous()` to return
   correct responses for deferred fields.
+- Fix Model instance non picklable GH-330
 
 3.1.2 (2018.05.09)
 ------------------

model_utils/tracker.py

@@ -207,6 +207,7 @@ class FieldTracker(object):
     def contribute_to_class(self, cls, name):
         self.name = name
         self.attname = '_%s' % name
+        self.patch_save(cls)
         models.signals.class_prepared.connect(self.finalize_class, sender=cls)
 
     def finalize_class(self, sender, **kwargs):
@@ -230,14 +231,13 @@ class FieldTracker(object):
         tracker = self.tracker_class(instance, self.fields, self.field_map)
         setattr(instance, self.attname, tracker)
         tracker.set_saved_fields()
-        self.patch_save(instance)
         instance._instance_intialized = True
 
-    def patch_save(self, instance):
-        original_save = instance.save
+    def patch_save(self, model):
+        original_save = model.save
 
-        def save(**kwargs):
-            ret = original_save(**kwargs)
+        def save(instance, **kwargs):
+            ret = original_save(instance, **kwargs)
             update_fields = kwargs.get('update_fields')
             if not update_fields and update_fields is not None:  # () or []
                 fields = update_fields
@@ -253,7 +253,7 @@ class FieldTracker(object):
             )
             return ret
 
-        instance.save = save
+        model.save = save
 
     def __get__(self, instance, owner):
         if instance is None:

tests/settings.py

@@ -8,3 +8,9 @@ DATABASES = {
     }
 }
 SECRET_KEY = 'dummy'
+
+CACHES = {
+    'default': {
+        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
+    }
+}

tests/test_fields/test_field_tracker.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 import django
 from django.core.exceptions import FieldError
 from django.test import TestCase
-
+from django.core.cache import cache
 from model_utils import FieldTracker
 from model_utils.tracker import DescriptorWrapper
 from tests.models import (
@@ -639,6 +639,16 @@ class ModelTrackerTests(FieldTrackerTests):
 
     tracked_class = ModelTracked
 
+    def test_cache_compatible(self):
+        cache.set('key', self.instance)
+        instance = cache.get('key')
+        instance.number = 1
+        instance.name = 'cached'
+        instance.save()
+        self.assertChanged()
+        instance.number = 2
+        self.assertHasChanged(number=True)
+
     def test_pre_save_changed(self):
         self.assertChanged()
         self.instance.name = 'new age'