Add support for model keys cast to backed enums in FormatModel

[saadsidqui]

As the title states, add support for model keys cast to backed enums in FormatModel. I have not scouted the whole codebase, there may be other places where a fix is needed to support backed enums for model keys.

I came across this bug while working on a project of mine. I had left this project alone for a couple of weeks before resuming it today. The issue didn't happen before, and I did a composer update when resuming, so this might be a regression. I, unfortunately, do not have the time to check more thoroughly.

Hope this helps. Thank you.

[saadsidqui]

This will probably break support for PHP < 8.1. I'm not sure how compatibility is handled in this project, any help would be greatly appreciated.

[driesvints]

We can't break apps on <8.1. 7.3 is the minimum version right now. You'll need to do a class_exists check I think.

[saadsidqui]

Updated. The reasoning is, if the interface_exists() check fails it will short-circuit, and the rest of the if condition will not execute. Thus, this should maintain compatibility with PHP < 8.1.

[saadsidqui]

I have just noticed also, it might be worthwhile to implement a similar check on the case at line 18. Let me know if that makes sense and I will refactor the whole thing to include all cases.

[driesvints]

Line 18 seems fine to me.

[saadsidqui]

I meant the block fulfilling the condition on line 18, specifically line 20 and 21. In that case, $model->getAttribute will also return the foreign/related key id cast to an enum, if such cast is set in the pivot instance. It makes sense given that $model->getKey on line 24 is just a shortcut for getAttribute.

In any case, I have provided the more complete solution that should take care of either case. LMK if you see any issues.