Ticket #18501: db.models.fields.related-fix-subclassed.patch

db/models/fields/related.py

@@ -124 +124 @@
         if not cls._meta.abstract:
             self.contribute_to_related_class(other, self.related)
 
+    def value_from_object(self, obj) :
+        """
+        If the foreign key is a subclassed field, we need to convert the value
+        to its python representation. This function is called by model_to_dict,
+        which presents the values as a dictionary to the form.
+        getattr() will put the database value on the dict by default as that is
+        what the real relationship represents. In the common (unsubclassed)
+        case, they will be the same value, however if subclassed this results
+        in:
+        a) get_prep_value on the custom model field being called with database
+           value. Working around this and just returning the value results in:
+        b) Select widget not being able to match the database representation
+           with the python representation.
+        """
+        return self.rel.get_related_field().to_python(getattr(obj, self.attname))
+
     def get_prep_lookup(self, lookup_type, value):
         if hasattr(value, 'prepare'):
             return value.prepare()