System check for default database values with expressions prohibits non-expressions

[Tim Graham]

Since its introduction in Django 5.0, ​the fields.E011 system check for database backends that have DatabaseFeatures.supports_expression_defaults = False ​requires literal defaults to be wrapped in Value.

There are a number of test models that have ​int, float and ​string db_defaults that will raise a system check error if DatabaseFeatures.supports_expression_defaults = False
since these models don't have required_db_features = {"supports_expression_defaults"}.

I'm working on MongoDB which doesn't support any database defaults, literal or expressions.

[Tim Graham]

I'm not sure if the appropriate solution is to add Value wrapping to all literal defaults in Django's test models or to modify the ​isinstance(db_default, Value) check to also accept int/float, str, etc.

[Lily Foote]

I think either solution could work, but maybe there's a subtlety I don't remember. I think wrapping all literals in Value is most likely to just work.

[Tim Graham]

The issue I see with requiring Value wrapping is that third-party apps may not know to do it and therefore run into compatibility issues with backends with supports_expression_defaults = False.​ For the other solution I suggested, I'm not sure we can make the isinstance()check exhaustive for all possible non-expression types.

Perhaps it's more appropriate to replace isinstance(self.db_default, Value) with not hasattr(self.db_default, "resolve_expression") to reject expressions.

[Tim Graham]

​PR to prevent the system check from rejecting Field.db_default literals (integer, float, boolean, etc.) when DatabaseFeatures.supports_expression_defaults = False.

[Sarah Boyce <42296566+sarahboyce@…>]

In c4614b5:

Fixed #35815 -- Made system check accept db_default literals when DatabaseFeatures.supports_expression_defaults = False.