Fixed #26524 -- Fixed crash in admin change view when displaying many to many forward refs.

Thanks Tim Graham for the regression test.

django/contrib/admin/utils.py

@@ -317,7 +317,7 @@ def _get_non_gfk_field(opts, name):
         raise FieldDoesNotExist()
 
     # Avoid coercing <FK>_id fields to FK
-    if field.is_relation and hasattr(field, 'attname') and field.attname == name:
+    if field.is_relation and not field.many_to_many and hasattr(field, 'attname') and field.attname == name:
         raise FieldIsAForeignKeyColumnName()
 
     return field

tests/admin_views/models.py

@@ -596,9 +596,13 @@ class CyclicTwo(models.Model):
         return self.name
 
 
+@python_2_unicode_compatible
 class Topping(models.Model):
     name = models.CharField(max_length=20)
 
+    def __str__(self):
+        return self.name
+
 
 class Pizza(models.Model):
     name = models.CharField(max_length=20)

tests/admin_views/tests.py

@@ -4696,6 +4696,14 @@ class ReadonlyTest(AdminFieldExtractionMixin, TestCase):
         response = self.client.get(reverse('admin:admin_views_topping_add'))
         self.assertEqual(response.status_code, 200)
 
+    def test_readonly_manytomany_forwards_ref(self):
+        topping = Topping.objects.create(name='Salami')
+        pizza = Pizza.objects.create(name='Americano')
+        pizza.toppings.add(topping)
+        response = self.client.get(reverse('admin:admin_views_pizza_change', args=(pizza.pk,)))
+        self.assertContains(response, '<label>Toppings:</label>', html=True)
+        self.assertContains(response, '<p>Salami</p>', html=True)
+
     def test_readonly_onetoone_backwards_ref(self):
         """
         Can reference a reverse OneToOneField in ModelAdmin.readonly_fields.