[1.6.x] 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.

Backpatch of 0e079e4331 from master.

tests/generic_relations_regress/models.py

@@ -123,8 +123,17 @@ class Tag(models.Model):
 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

tests/generic_relations_regress/tests.py

@@ -1,6 +1,7 @@
 from django.db.models import Q
 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,
@@ -212,3 +213,13 @@ class GenericRelationTests(TestCase):
         # B would then fail).
         self.assertNotIn(" join ", str(B.objects.exclude(a__flag=True).query).lower())
         self.assertIn("content_type_id", str(B.objects.exclude(a__flag=True).query).lower())
+
+    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)