Fixed #25550 -- Deprecated direct assignment to the reverse side of a related set.

django/core/serializers/base.py

@@ -201,7 +201,7 @@ class DeserializedObject(object):
         models.Model.save_base(self.object, using=using, raw=True, **kwargs)
         if self.m2m_data and save_m2m:
             for accessor_name, object_list in self.m2m_data.items():
-                setattr(self.object, accessor_name, object_list)
+                getattr(self.object, accessor_name).set(object_list)
 
         # prevent a second (possibly accidental) call to save() from saving
         # the m2m data twice.

django/db/models/fields/related.py

@@ -1600,7 +1600,7 @@ class ManyToManyField(RelatedField):
         return getattr(obj, self.attname).all()
 
     def save_form_data(self, instance, data):
-        setattr(instance, self.attname, data)
+        getattr(instance, self.attname).set(data)
 
     def formfield(self, **kwargs):
         db = kwargs.pop('using', None)

django/db/models/fields/related_descriptors.py

@@ -62,11 +62,13 @@ and two directions (forward and reverse) for a total of six combinations.
 
 from __future__ import unicode_literals
 
+import warnings
 from operator import attrgetter
 
 from django.db import connections, router, transaction
 from django.db.models import Q, signals
 from django.db.models.query import QuerySet
+from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.functional import cached_property
 
 
@@ -477,6 +479,11 @@ class ReverseManyToOneDescriptor(object):
         - ``instance`` is the ``parent`` instance
         - ``value`` in the ``children`` sequence on the right of the equal sign
         """
+        warnings.warn(
+            'Direct assignment to the reverse side of a related set is '
+            'deprecated due to the implicit save() that happens. Use %s.set() '
+            'instead.' % self.rel.get_accessor_name(), RemovedInDjango20Warning, stacklevel=2,
+        )
         manager = self.__get__(instance)
         manager.set(value)