Fixed #18949 -- Improve performance of model_to_dict with many-to-many

When calling model_to_dict, improve performance of the generated SQL by
using values_list to determine primary keys of many to many objects. Add
a specific test for this function, test_model_to_dict_many_to_many

Thanks to brian for the original report and suggested fix.

django/forms/models.py

@@ -126,7 +126,7 @@ def model_to_dict(instance, fields=None, exclude=None):
                 data[f.name] = []
             else:
                 # MultipleChoiceWidget needs a list of pks, not object instances.
-                data[f.name] = [obj.pk for obj in f.value_from_object(instance)]
+                data[f.name] = list(f.value_from_object(instance).values_list('pk', flat=True))
         else:
             data[f.name] = f.value_from_object(instance)
     return data

tests/modeltests/model_forms/tests.py

@@ -561,6 +561,42 @@ class UniqueTest(TestCase):
             "slug": "Django 1.0"}, instance=p)
         self.assertTrue(form.is_valid())
 
+class ModelToDictTests(TestCase):
+    """
+    Tests for forms.models.model_to_dict
+    """
+    def test_model_to_dict_many_to_many(self):
+        categories=[
+            Category(name='TestName1', slug='TestName1', url='url1'),
+            Category(name='TestName2', slug='TestName2', url='url2'),
+            Category(name='TestName3', slug='TestName3', url='url3')
+        ]
+        for c in categories:
+            c.save()
+        writer = Writer(name='Test writer')
+        writer.save()
+
+        art = Article(
+            headline='Test article',
+            slug='test-article',
+            pub_date=datetime.date(1988, 1, 4),
+            writer=writer,
+            article='Hello.'
+        )
+        art.save()
+        for c in categories:
+            art.categories.add(c)
+        art.save()
+
+        with self.assertNumQueries(1):
+            d = model_to_dict(art)
+
+        #Ensure all many-to-many categories appear in model_to_dict
+        for c in categories:
+            self.assertIn(c.pk, d['categories'])
+        #Ensure many-to-many relation appears as a list
+        self.assertIsInstance(d['categories'], list)
+
 class OldFormForXTests(TestCase):
     def test_base_form(self):
         self.assertEqual(Category.objects.count(), 0)