Update dal.py

updated to handle references without format

gluon/dal.py

@@ -51,9 +51,11 @@ def _default_validators(db, field):
         if hasattr(referenced, '_format') and referenced._format:
             requires = validators.IS_IN_DB(db, referenced._id,
                                            referenced._format)
+        else:
+            requires = validators.IS_IN_DB(db, referenced._id)
         if field.unique:
             requires._and = validators.IS_NOT_IN_DB(db, field)
-            if field.tablename == field_type[10:]:
+        if not field.notnull:
             return validators.IS_EMPTY_OR(requires)
         return requires
     elif db and field_type.startswith('list:reference') and \