Model.full_clean() does not recalculate GeneratedFields prior to validating model constraints

[Mark Gensler]

GeneratedFields on a model instance are not recalculated during Model.full_clean(). Therefore, if a GeneratedField is included in the *expressions or fields parameters of a UniqueConstraint or CheckConstraint in the instance's Model._meta, the new values of the GeneratedField attribute are not uaed by BaseConstraint.validate().

Instead, the model instance's GeneratedField attribute will return the value which corresponds to the most recent refresh of the instance from the database. If the instance is new and has not yet been saved, the following error is raised:

AttributeError: Cannot read a generated field from an unsaved model.

An IntegrityError is correctly raised when calling Model.save() if any constraints involving GeneratedFields are violated.

Instead, the recalculated values for GeneratedFields should be used when checking constraints.

This ticket was raised after fixing the bug in #35560 with PR ​https://github.com/django/django/pull/18309.

[Mark Gensler]

I'm happy to start working on this. I will raise a PR and add failing tests to catch the bug shortly.

Before writing any code to address this, I'd like to discuss where best to apply the fix. I see two possible options:

1. Recalculate GeneratedField attribute values on the fly as part of calling Constraint.validate(). This will leave the instance attributes unchanged until Model.save() is called.
2. Recalculate GeneratedField attributes on the model instance during Model.full_clean(), prior to calls to Constraint.validate(), and irrespective of whether relevant constraints are present.

The second option would allow further checks against the new GeneratedField value of the instance to be made during validation. On the other hand, the first option would leave the instance's attribute returning the value currently returned/stored by the database.

Which would be best?