[9.x] Fix loadAggregate not correctly applying casts

[aalyusuf]

Reopening #41025 with tests and better explanation about the bug.

There's a bug in loadAggregate in Eloquent Collections specefically when running LoadExists where running something like

$user->articles->loadExists("comments") will add comments_exists to all existing articles but only the first model will have bool casts automatically added to it. Resulting in something similar to:

.. "comments_exists" => true
.. "comments_exists" => '1'
.. "comments_exists" => '1'  

This PR fixes that and adds casts to all items instead of only the first one. I highly doubt it'll be breaking change to anyone.

[taylorotwell]

Can you provide a simple example of how I can recreate this issue in a fresh Laravel application with code examples? Please mark this as ready for review when you have done so otherwise I won't see it.

[aalyusuf]

Hey @taylorotwell

Here's a simple code example ( Also the PR contains a test which would fail on a fresh laravel installation ):

Models

class User extends Authenticatable
{

    public function posts()
    {
        return $this->hasMany(Post::class);
    }

}

class Post extends Model
{
    use HasFactory;
    protected $guarded = [];

    public function comments()
    {
        return $this->hasMany(Comment::class);
    }
}

class Comment extends Model
{
    use HasFactory;
}

Some data

    $user = \App\Models\User::find(1);
    $user->posts()->createMany([
        ['content' => 'Test'],
        ['content' => 'Test'],
        ['content' => 'Test'],
    ]);

The test
Now, running loadExists on comments

    $user = \App\Models\User::with('posts')->find(1);
    $user->posts->loadExists("comments");
    return $user->posts;

This will result in the following ( notice comments_exists ) , loadExists here will only cast the first Post we created

and

 dd($user->posts->pluck('comments_exists')); 

[taylorotwell]

I created a user with 3 posts and each posts has 3 comments... this is what I get:

[aalyusuf]

I believe that's the response because of tinker & it's not casting there. But after executing loadExists the next line you can try can be:

$user->posts->pluck('comments_exists');

and this should reproduce the issue

or even directly

$user->posts->map->getCasts();

should give you:

[taylorotwell]

Thanks