Refresh fields that are expressions after Model.save()

[holvianssi]

The use case is automatically fetching the value for expressions when saving to DB. For example:

> user = User.objects.create(username=Lower('Anssi'))
> user.username == 'anssi'
True

While the above feature can be simulated somewhat easily by calling refresh_from_db() after save, an in-built implementation has the ability to use RETURNING as an optimization. In addition, it seems that refreshing objects on save would be a nice default, though this might be a bit backwards incompatible in some cases.

[Tim Graham]

Does this also fix #23386? In that case, we might close this as a duplicate.

[Simon Charette]

​PR

[Simon Charette]

This might also be related to #21454 or at least to the implementation proposed in it's ​PR as we should really be using RETURNING on backends that support it instead of performing a second SELECT to fetch the database generated fields.

[holvianssi]

I'm hesitant to go with RETURNING implementation for the first patch. The select approach is really simple, and it will be needed in any case for some backends. The RETURNING approach on the other hand will be complex, and after all it's just an optimisation.

This should almost solve #21454 with a Default expression. This would need to be assigned manually to fields pre-save. Then full solution to #21454 would be adding a bit of API to do the pre-save assignments automatically.

And yes, I believe this solves #23386.

[Tim Graham]

Comments for improvement on the PR.

[Tim Graham]

I update the PR per my comments.

[Tim Graham]

Simon still has concerns described on the pull request.

[Simon Charette]

Updated ​patch that makes use of RETURNING on backends that support it and clear the attributes otherwise (allowing refresh_from_db to kick in on attribute access). Still needs a few tweaks.