Use a signal handler instead of patching save.

References #83.

Instead of patching the save method of a tracked model class, we can use
a signal handler on post_save, which means we can still pickle our model
class.

Note we can't just listen for the signal from the class we have, but
instead listen for all post_save signals. This means we actually install
a new signal handler for each tracked model class, which fires on all
model save occurrences (and returns immediately if this handler doesn't care).

We probably could improve this to have a registry of tracked models, or
something, that allows us to just install one signal handler, and filter
according to membership.

model_utils/tests/tests.py

@@ -1497,6 +1497,9 @@ class FieldTrackerTests(FieldTrackerTestCase, FieldTrackerCommonTests):
 
         item.number = 2
         self.assertTrue(item.tracker.has_changed('number'))
+    
+    def test_can_pickle_objects(self):
+        pickle.dumps(self.instance)
 
 
 class FieldTrackedModelCustomTests(FieldTrackerTestCase,

model_utils/tracker.py

@@ -2,10 +2,11 @@ from __future__ import unicode_literals
 
 from copy import deepcopy
 
-from django.db import models
 from django.core.exceptions import FieldError
+from django.db import models
 from django.db.models.query_utils import DeferredAttribute
-
+from django.db.models.signals import post_save
+from django.dispatch import receiver
 
 class FieldInstanceTracker(object):
     def __init__(self, instance, fields, field_map):
@@ -119,20 +120,16 @@ class FieldTracker(object):
         models.signals.post_init.connect(self.initialize_tracker)
         self.model_class = sender
         setattr(sender, self.name, self)
-
-    def initialize_tracker(self, sender, instance, **kwargs):
-        if not isinstance(instance, self.model_class):
-            return  # Only init instances of given model (including children)
-        tracker = self.tracker_class(instance, self.fields, self.field_map)
-        setattr(instance, self.attname, tracker)
-        tracker.set_saved_fields()
-        self.patch_save(instance)
-
-    def patch_save(self, instance):
-        original_save = instance.save
-        def save(**kwargs):
-            ret = original_save(**kwargs)
+        
+        # Rather than patch the save method on the instance,
+        # we can observe the post_save signal on the class.
+        @receiver(post_save, sender=None, weak=False)
+        def handler(sender, instance, **kwargs):
+            if not isinstance(instance, self.model_class):
+                return
+            
             update_fields = kwargs.get('update_fields')
+            
             if not update_fields and update_fields is not None:  # () or []
                 fields = update_fields
             elif update_fields is None:
@@ -142,12 +139,19 @@ class FieldTracker(object):
                     field for field in update_fields if
                     field in self.fields
                 )
+            
             getattr(instance, self.attname).set_saved_fields(
                 fields=fields
             )
-            return ret
-        instance.save = save
+        
 
+    def initialize_tracker(self, sender, instance, **kwargs):
+        if not isinstance(instance, self.model_class):
+            return  # Only init instances of given model (including children)
+        tracker = self.tracker_class(instance, self.fields, self.field_map)
+        setattr(instance, self.attname, tracker)
+        tracker.set_saved_fields()
+        
     def __get__(self, instance, owner):
         if instance is None:
             return self