Refs #24227 -- Removed ManyToManyField special casing in model_to_dict().

django/db/models/fields/related.py

@@ -1565,7 +1565,10 @@ class ManyToManyField(RelatedField):
         """
         Return the value of this field in the given model instance.
         """
-        return getattr(obj, self.attname).all()
+        qs = getattr(obj, self.attname).all()
+        if qs._result_cache is not None:
+            return [item.pk for item in qs]
+        return list(qs.values_list('pk', flat=True))
 
     def save_form_data(self, instance, data):
         getattr(instance, self.attname).set(data)

django/forms/models.py

@@ -88,21 +88,7 @@ def model_to_dict(instance, fields=None, exclude=None):
             continue
         if exclude and f.name in exclude:
             continue
-        if f.many_to_many:
-            # If the object doesn't have a primary key yet, just use an empty
-            # list for its m2m fields. Calling f.value_from_object will raise
-            # an exception.
-            if instance.pk is None:
-                data[f.name] = []
-            else:
-                # MultipleChoiceWidget needs a list of pks, not object instances.
-                qs = f.value_from_object(instance)
-                if qs._result_cache is not None:
-                    data[f.name] = [item.pk for item in qs]
-                else:
-                    data[f.name] = list(qs.values_list('pk', flat=True))
-        else:
-            data[f.name] = f.value_from_object(instance)
+        data[f.name] = f.value_from_object(instance)
     return data