Add reset_polymorphic_ctype() function to assist with migration to polymorphic

polymorphic/utils.py

@@ -0,0 +1,22 @@
+from django.contrib.contenttypes.models import ContentType
+
+
+def reset_polymorphic_ctype(*models, **filters):
+    """
+    Set the polymorphic content-type ID field to the proper model
+    Sort the ``*models`` from base class to descending class,
+    to make sure the content types are properly assigned.
+
+    Add ``preserve_existing=True`` to skip models which already
+    have a polymorphic content type.
+    """
+    preserve_existing = filters.pop('preserve_existing', False)
+    for new_model in models:
+        new_ct = ContentType.objects.get_for_model(new_model)
+
+        qs = new_model.objects.all()
+        if preserve_existing:
+            qs = qs.filter(polymorphic_ctype__isnull=True)
+        if filters:
+            qs = qs.filter(**filters)
+        qs.update(polymorphic_ctype=new_ct)