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.

5 files changed, 35 insertions, 6 deletions

diff --git a/django/contrib/contenttypes/generic.py b/django/contrib/contenttypes/generic.py
index 4880214200..469e64db33 100644
--- a/django/contrib/contenttypes/generic.py
+++ b/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
diff --git a/django/forms/models.py b/django/forms/models.py
index 6d9a567926..610a2a176a 100644
--- a/django/forms/models.py
+++ b/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
diff --git a/docs/releases/1.6.1.txt b/docs/releases/1.6.1.txt
index baf53e5cd3..55115704bb 100644
--- a/docs/releases/1.6.1.txt
+++ b/docs/releases/1.6.1.txt
@@ -20,3 +20,5 @@ Bug fixes
 * Fixed :class:`~django.contrib.auth.backends.ModelBackend` raising
   ``UnboundLocalError`` if :func:`~django.contrib.auth.get_user_model`
   raised an error (#21439).
+* Fixed a regression that prevented editable ``GenericRelation`` subclasses
+  from working in ``ModelForms``.
diff --git a/tests/generic_relations_regress/models.py b/tests/generic_relations_regress/models.py
index 300ed9d54a..e9fb41dfe3 100644
--- a/tests/generic_relations_regress/models.py
+++ b/tests/generic_relations_regress/models.py
@@ -143,8 +143,18 @@ class Board(models.Model):
     name = models.CharField(primary_key=True, max_length=15)
 
 
+class SpecialGenericRelation(generic.GenericRelation):
+    def __init__(self, *args, **kwargs):
+        super(SpecialGenericRelation, self).__init__(*args, **kwargs)
+        self.editable = True
+        self.save_form_data_calls = 0
+
+    def save_form_data(self, *args, **kwargs):
+        self.save_form_data_calls += 1
+
+
 class HasLinks(models.Model):
-    links = generic.GenericRelation(Link)
+    links = SpecialGenericRelation(Link)
 
     class Meta:
         abstract = True
diff --git a/tests/generic_relations_regress/tests.py b/tests/generic_relations_regress/tests.py
index 285bd43c47..6e6ada5872 100644
--- a/tests/generic_relations_regress/tests.py
+++ b/tests/generic_relations_regress/tests.py
@@ -1,6 +1,7 @@
 from django.db.models import Q, Sum
 from django.db.utils import IntegrityError
 from django.test import TestCase, skipIfDBFeature
+from django.forms.models import modelform_factory
 
 from .models import (
     Address, Place, Restaurant, Link, CharLink, TextLink,
@@ -236,3 +237,13 @@ class GenericRelationTests(TestCase):
         # Finally test that filtering works.
         self.assertEqual(qs.filter(links__sum__isnull=True).count(), 1)
         self.assertEqual(qs.filter(links__sum__isnull=False).count(), 0)
+
+    def test_editable_generic_rel(self):
+        GenericRelationForm = modelform_factory(HasLinkThing, fields='__all__')
+        form = GenericRelationForm()
+        self.assertIn('links', form.fields)
+        form = GenericRelationForm({'links': None})
+        self.assertTrue(form.is_valid())
+        form.save()
+        links = HasLinkThing._meta.get_field_by_name('links')[0].field
+        self.assertEqual(links.save_form_data_calls, 1)