Retrieving eloquent api resource using keyby collection method

Question

I have an end API point

users/{user}

now in User resource, I want to return

     public function toArray($request)
        {
            // return parent::toArray($request);

            return [
                'id' => $this->id,
                'name' => $this->name,
//                'comments' => $this->post->comments->keyBy('post_id')
                'comments' => new CommentCollection($this->post->comments->keyBy->post_id)

            ];
        }

CommentCollection class

public function toArray($request)
    {
        // return parent::toArray($request);

        return [
            'data' => $this->collection->transform(function($comment){
                return [
                    'id' => $comment->id,
                    'comment' => $comment->comment,
                ];
            }),
        ];
    }

but the result will not include the post_id as key, how I can make it return the comments collection having key post_id?

Update

use App\models\Post;
use App\Http\Resources\Postas PostResource;

Route::get('/posts', function () {
    return PostResource::collection(Post::all()->keyBy->slug);
});

This is working correctly, but if I will use post collection inside User resource as relationship, it is not working! and that is my requirement in comments collection.

Answer 1

What I did it, I created another ResourceGroupCollection class

<?php
namespace App\Http\Resources\Collection;

use Illuminate\Http\Resources\Json\ResourceCollection;

class CommentGroupCollection extends ResourceCollection
{
    public $collects = 'App\Http\Resources\Collection\CommentCollection';

    public $preserveKeys = true;

    public function toArray($request)
    {
        return $this->collection;
    }

}

<?php
namespace App\Http\Resources\Collection;

use Illuminate\Http\Resources\Json\ResourceCollection;

class CommentCollection extends ResourceCollection
{
    public $collects = 'App\Http\Resources\Comment';

    public $preserveKeys = true;

    public function toArray($request)
    {
        return $this->collection;
    }

}

and then 

new CommentGroupCollection($comments->groupBy('post_id')),