Explain how to access attachments before save

[ahmeij]

Summary

Explain how to access attachments before save since the files will not be sent to storage anymore before the model is saved. This was changed here: #33303

[ahmeij]

Sorry, I added [ci skip] to the comment above, not the commit message.

[georgeclaghorn]

attachment_changes is intentionally internal API. If we want to make it possible to access attachables before save, we should expose a nicer public API for it.

[ahmeij]

Good feedback. I would've liked the chance to update this change request with code to expose this functionality, but reading between the lines I am unsure if that would be merged.

I do see however a clear reason for having access to the uploaded file before save. The current setup makes ActiveStorage a bit weird, all changes to the record are visible during validation and before_save, the attachments are becoming the only exception.

Please let know if something along the lines of avatar_file = record.avatar.uploaded is something worth creating a new pull-request for.

[georgeclaghorn]

Please do open a new PR (though I can’t promise that it’ll be accepted sight unseen).