This should probably be a separate method -- perhaps, one that could be used for the insert as well.
The code here should read,
```python
if self.returning_fields:
    r_sql, r_params = self.return_values_fragment()
    if r_sql: 
       result.append(r_sql)
       params += r_params
```

The default `return_values_fragment()` should raise `django.db.utils.NotSupportedError`