Fixed #21428 -- editable GenericRelation regression

The GenericRelation refactoring removed GenericRelations from
model._meta.many_to_many. This had the side effect of disallowing
editable GenericRelations in ModelForms. Editable GenericRelations
aren't officially supported, but if we don't fix this we don't offer any
upgrade path for those who used the ability to set editable=True
in GenericRelation subclass.

Thanks to Trac alias joshcartme for the report and stephencmd and Loic
for working on this issue.

django/contrib/contenttypes/generic.py

@@ -39,6 +39,7 @@ class GenericForeignKey(six.with_metaclass(RenameGenericForeignKeyMethods)):
         self.ct_field = ct_field
         self.fk_field = fk_field
         self.for_concrete_model = for_concrete_model
+        self.editable = False
 
     def contribute_to_class(self, cls, name):
         self.name = name

django/forms/models.py

@@ -83,7 +83,12 @@ def save_instance(form, instance, fields=None, fail_message='saved',
     # Wrap up the saving of m2m data as a function.
     def save_m2m():
         cleaned_data = form.cleaned_data
-        for f in opts.many_to_many:
+        # Note that for historical reasons we want to include also
+        # virtual_fields here. (GenericRelation was previously a fake
+        # m2m field).
+        for f in opts.many_to_many + opts.virtual_fields:
+            if not hasattr(f, 'save_form_data'):
+                continue
             if fields and f.name not in fields:
                 continue
             if exclude and f.name in exclude:
@@ -119,8 +124,8 @@ def model_to_dict(instance, fields=None, exclude=None):
     from django.db.models.fields.related import ManyToManyField
     opts = instance._meta
     data = {}
-    for f in opts.concrete_fields + opts.many_to_many:
-        if not f.editable:
+    for f in opts.concrete_fields + opts.virtual_fields + opts.many_to_many:
+        if not getattr(f, 'editable', False):
             continue
         if fields and not f.name in fields:
             continue
@@ -174,8 +179,8 @@ def fields_for_model(model, fields=None, exclude=None, widgets=None,
     field_list = []
     ignored = []
     opts = model._meta
-    for f in sorted(opts.concrete_fields + opts.many_to_many):
-        if not f.editable:
+    for f in sorted(opts.concrete_fields + opts.virtual_fields + opts.many_to_many):
+        if not getattr(f, 'editable', False):
             continue
         if fields is not None and not f.name in fields:
             continue