SDE-5196 Save queries when checking m2m fields

Add flag to for simple m2m tracking, saving us DB queries

src/dirtyfields/dirtyfields.py

@@ -17,6 +17,11 @@ class DirtyFieldsMixin(object):
     # https://github.com/romgar/django-dirtyfields/issues/73
     ENABLE_M2M_CHECK = False
 
+    # Flag to track M2M changes as a simple boolean; less accurate than
+    # ENABLE_M2M_CHECK as we're only tracking if m2m relations were updated,
+    # not whether they are dirty
+    ENABLE_BASIC_M2M_CHECK = False
+
     FIELDS_TO_CHECK = None
 
     def __init__(self, *args, **kwargs):
@@ -25,14 +30,15 @@ def __init__(self, *args, **kwargs):
             reset_state, sender=self.__class__, weak=False,
             dispatch_uid='{name}-DirtyFieldsMixin-sweeper'.format(
                 name=self.__class__.__name__))
-        if self.ENABLE_M2M_CHECK:
+        if self.ENABLE_M2M_CHECK or self.ENABLE_BASIC_M2M_CHECK:
             self._connect_m2m_relations()
         reset_state(sender=self.__class__, instance=self)
 
     def _connect_m2m_relations(self):
+        m2m_handler = set_m2m_dirty if self.ENABLE_BASIC_M2M_CHECK else reset_state
         for m2m_field, model in get_m2m_with_model(self.__class__):
             m2m_changed.connect(
-                reset_state, sender=remote_field(m2m_field).through, weak=False,
+                m2m_handler, sender=remote_field(m2m_field).through, weak=False,
                 dispatch_uid='{name}-DirtyFieldsMixin-sweeper-m2m'.format(
                     name=self.__class__.__name__))
 
@@ -121,6 +127,8 @@ def get_dirty_fields(self, check_relationship=False, check_m2m=None, verbose=Fal
         return modified_fields
 
     def is_dirty(self, check_relationship=False, check_m2m=None):
+        if self.ENABLE_BASIC_M2M_CHECK and self._m2m_dirty:
+            return True
         return {} != self.get_dirty_fields(check_relationship=check_relationship,
                                            check_m2m=check_m2m)
 
@@ -164,3 +172,9 @@ def reset_state(sender, instance, **kwargs):
         instance._original_state = new_state
     if instance.ENABLE_M2M_CHECK:
         instance._original_m2m_state = instance._as_dict_m2m()
+    elif instance.ENABLE_BASIC_M2M_CHECK:
+        instance._m2m_dirty = False
+
+
+def set_m2m_dirty(sender, instance, **kwargs):
+    instance._m2m_dirty = True