Add an option to deactivate m2m checks, if not needed

Author: romgar

src/dirtyfields/dirtyfields.py

@@ -12,13 +12,18 @@
 class DirtyFieldsMixin(object):
     compare_function = (raw_compare, {})
 
+    # This mode has been introduced to handle some situations like this one:
+    # https://github.com/romgar/django-dirtyfields/issues/73
+    enable_m2m_check = True
+
     def __init__(self, *args, **kwargs):
         super(DirtyFieldsMixin, self).__init__(*args, **kwargs)
         post_save.connect(
             reset_state, sender=self.__class__,
             dispatch_uid='{name}-DirtyFieldsMixin-sweeper'.format(
                 name=self.__class__.__name__))
-        self._connect_m2m_relations()
+        if self.enable_m2m_check:
+            self._connect_m2m_relations()
         reset_state(sender=self.__class__, instance=self)
 
     def _connect_m2m_relations(self):
@@ -83,6 +88,9 @@ def get_dirty_fields(self, check_relationship=False, check_m2m=None, verbose=Fal
             initial_dict = self._as_dict(check_relationship, include_primary_key=False)
             return initial_dict
 
+        if check_m2m is not None and not self.enable_m2m_check:
+            raise Exception("You can't check m2m fields if enable_m2m_check is set to False")
+
         modified_fields = compare_states(self._as_dict(check_relationship),
                                          self._original_state,
                                          self.compare_function)
@@ -112,4 +120,5 @@ def reset_state(sender, instance, **kwargs):
     # original state should hold all possible dirty fields to avoid
     # getting a `KeyError` when checking if a field is dirty or not
     instance._original_state = instance._as_dict(check_relationship=True)
-    instance._original_m2m_state = instance._as_dict_m2m()
+    if instance.enable_m2m_check:
+        instance._original_m2m_state = instance._as_dict_m2m()