Fixed #29078 -- Made serializers respect prefetch_related() for m2m fields.

django/core/serializers/python.py

@@ -67,9 +67,11 @@ class Serializer(base.Serializer):
             else:
                 def m2m_value(value):
                     return self._value_from_field(value, value._meta.pk)
-            self._current[field.name] = [
-                m2m_value(related) for related in getattr(obj, field.name).iterator()
-            ]
+            m2m_iter = getattr(obj, '_prefetched_objects_cache', {}).get(
+                field.name,
+                getattr(obj, field.name).iterator(),
+            )
+            self._current[field.name] = [m2m_value(related) for related in m2m_iter]
 
     def getvalue(self):
         return self.objects

django/core/serializers/xml_serializer.py

@@ -132,7 +132,11 @@ class Serializer(base.Serializer):
                     self.xml.addQuickElement("object", attrs={
                         'pk': str(value.pk)
                     })
-            for relobj in getattr(obj, field.name).iterator():
+            m2m_iter = getattr(obj, '_prefetched_objects_cache', {}).get(
+                field.name,
+                getattr(obj, field.name).iterator(),
+            )
+            for relobj in m2m_iter:
                 handle_m2m(relobj)
 
             self.xml.endElement("field")

tests/serializers/tests.py

@@ -250,6 +250,19 @@ class SerializersTestBase:
         with self.assertNumQueries(0):
             serializers.serialize(self.serializer_name, [mv])
 
+    def test_serialize_prefetch_related_m2m(self):
+        # One query for the Article table and one for each prefetched m2m
+        # field.
+        with self.assertNumQueries(3):
+            serializers.serialize(
+                self.serializer_name,
+                Article.objects.all().prefetch_related('categories', 'meta_data'),
+            )
+        # One query for the Article table, and two m2m queries for each
+        # article.
+        with self.assertNumQueries(5):
+            serializers.serialize(self.serializer_name, Article.objects.all())
+
     def test_serialize_with_null_pk(self):
         """
         Serialized data with no primary key results