fix(laravel): Allow LinksHandler to handle polymorphic relationships

[jonerickson]

Q	A
Branch?	4.1
Tickets	#7227
License	MIT
Doc PR	api-platform/docs#...

This PR adds support for polymorphic relationships from within the LinksHandler.

Previously:

This would result in bad query formation where the model type was not taken into account and you could have multiple model types returned with the same IDs:.

select * from `attachments` where `attachments`.`model_id` = ?

new GetCollection(
    uriTemplate: '/projects/{id}/attachments',
    uriVariables: [
        'id' => new Link(
            fromProperty: 'attachments',
            fromClass: Project::class,
        ),
    ],
),

Now:

select * from `attachments` where `attachments`.`model_type` = ? and `attachments`.`model_id` = ? and `attachments`.`model_id` is not null

We now properly scope the query to the model class as well.

[jonerickson]

@soyuka in the first commit, I addressed the underlying issue. In the second commit, I am posposing a better way to handle links - using the built in standard eloquent relationship methods. By simply hydrating the base class with its identifier $relatedInstance->setAttribute($relatedInstance->getKeyName(), $identifier); we can then just call the relationship methods to get the querys - no longer have to build them from scratch. With BelongsTo we obviously have a different situation because the FK lives on the child table so we have to use native joins. I feel this is a much cleaner way moving forward.

[soyuka]

This line makes me think about #7166, its another subject but do you have any idea on how we could do otherwise to instantiate this class?

[jonerickson]

I think what needs to happen in the item/collection provider or property accessor is something like loadMissing on the relations that are discovered through the eloquent property metadata service. This error happens when calling a relation that hasn't already been hydrated in the class. By calling loadMissing we can hopefully prevent that. I'll play around with it and see what I can come up with.

[jonerickson]

Actually, we don't need to do anything anymore. This switched fix #7166 as well. See #7166 (comment)

[soyuka]

This is awesome thank you very much @jonerickson !