Shims to fix test collection errors

lms/djangoapps/courseware/models.py

@@ -257,14 +257,9 @@ class XBlockFieldBase(models.Model):
     modified = models.DateTimeField(auto_now=True, db_index=True)
 
     def __unicode__(self):
-        return u'{}<{!r}'.format(
-            self.__class__.__name__,
-            {
-                key: getattr(self, key)
-                for key in self._meta.get_all_field_names()
-                if key not in ('created', 'modified')
-            }
-        )
+        # pylint: disable=protected-access
+        keys = [field.name for field in self._meta.get_fields() if field.name not in ('created', 'modified')]
+        return u'{}<{!r}'.format(self.__class__.__name__, {key: getattr(self, key) for key in keys})
 
 
 class XModuleUserStateSummaryField(XBlockFieldBase):