Viability of Enhancing GeneratedField Updates in Django

Based on the available information about Django's model saving mechanism and the GeneratedField feature introduced in Django 5.0, your proposed enhancement appears conceptually viable, though it would require modifications to Django's current model-save paradigm.

Current Behavior

The GeneratedField in Django 5.0 allows for database-generated columns that compute their values based on other fields. As you correctly noted, when updating a model instance:

1. The generated field values are not automatically refreshed in the in-memory instance after save()
2. A manual refresh_from_db() call is required to see the updated values

Proposed Enhancement

Your suggestion to leverage the SQL RETURNING clause to update the in-memory instance during save() is technically sound. The two approaches you proposed:

1. Using update_fields=["side", "area"] to include generated fields
2. Adding a new returning_fields=["area"] parameter

Both would allow Django to fetch the updated generated values without requiring a separate database query.

Feasibility Analysis

The current update_fields parameter in Django's save() method is primarily used to specify which fields should be updated in the database. Based on the documentation:

PYTHONCopied
product.name = "Name changed again"
product.save(update_fields=["name"])

This mechanism is designed to optimize performance by limiting the fields included in the UPDATE statement.

Your proposal would extend this concept to also fetch certain fields' values after the update. This is conceptually different from the current purpose of update_fields, which might make the first approach less intuitive.

The second approach with a dedicated returning_fields parameter seems more explicit and aligned with the intended purpose.

Implementation Considerations

1. Django would need to modify its SQL generation for the save() method to include the RETURNING clause when appropriate
2. The ORM would need logic to update the in-memory instance with the returned values
3. Database compatibility would need to be considered, as not all databases support the RETURNING clause in the same way

Conclusion

The enhancement you're proposing is technically viable and would improve the developer experience when working with GeneratedField. The returning_fields approach seems more semantically appropriate than overloading the meaning of update_fields.

This would be a valuable feature request for the Django project, as it aligns with Django's philosophy of making common operations easy and explicit while reducing unnecessary database queries.