[8.x] Alias eloquent with-relationships

[crissi]

This pr enables relationships to be loaded with an alias name.

this allows you to use the same relationship multiple times with different constraints which is not possible today. Today you will have to add new relationships to the model to accomplish this.

This aligns the with-method with the withCount-method which supports aliases today: https://laravel.com/docs/7.x/eloquent-relationships#counting-related-models

For me personally I have a need for this with a graphql library that uses eloquent to create eager loaded queries. Since graphql supports aliases I would need to create different versions of relationship if an alias is used with different params that might change the constraints.

$user = EloquentTestUser::with([
            'posts as aliasPosts' => function($q) {
                $q->where('name', 'Second Post');
            },
            'posts.childPosts' => function($q) {
            },
            'posts.childPosts AS aliasChildPosts' => function($q) {
                $q->where('name', 'Child Post2');
            }
        ])->first();

[GrahamCampbell]

Please update the phpdoc for the functions whose signatures you have changed.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.

[PaolaRuby]

Closed only for not update the phpdoc?