Select specific column in laravel eloquent's with method

I am using laravel 5.6 and i want to get user posts with comments (only id field)

User Model

public function posts()
{
    return $this->hasMany('App\Post');
}

Post Model

public function user()
{
    return $this->belongsTo('App\User');
}

public function comments()
{
    return $this->hasMany('App\Comment');
}

Comment Model

public function post()
{
    return $this->belongsTo('App\Post');
}

In my controller i am using this code to get user posts with their comments

$posts = $request->user()->posts()->with(['comments' => function($query) {
    $query->select(['id']);
}]); 

But its not working...

When i comment $query->select(['id']); it works fine but returns Comment model all fields. I want to only select id field.

What i am missing here?