[9.x] Add whenCounted to JsonResource

[stevebauman]

Description

This PR adds a whenCounted method to the JSON resource class giving us the ability to conditionally include attributes in a response when a relation count is set on the model via Post::withCount('comments').

This method acts nearly identically to whenLoaded, with the exception that the retrieved count attribute value will be passed into the $value callback (if one is provided).

I've also added a hasAttribute method to models to easily determine if an attribute exists inside of the models raw $attributes array, so I didn't have to fetch all of the models attributes to determine if it exists:

if (! isset($this->resource->getAttributes()[$attribute])) {
    // ...
}

Example

class PostController
{
    public function show(Post $post)
    {
        return new PostResource($post->loadCount('comments'));
    }
}

class PostResource extends PostResource
{
    public function toArray($request)
    {
        return [
            'id' => $this->id,
            'comments_count' => $this->whenCounted('comments'),
        ];
    }
}

Important

If you'd prefer not having the hasAttribute method added onto Eloquent models, let me know and I can remove it!

Thanks for your time! ❤️

[taylorotwell]

@stevebauman yeah, you can remove that hasAttribute method from Eloquent.

[taylorotwell]

Can you PR docs for this @stevebauman ?

[stevebauman]

Sure thing @taylorotwell 👍