txt CommentRes:
public function toArray($request)
{
return ['user' =>
 new UserRes($this->loadMissing('user'))
 ];
    }

UserRes:
public function toArray($request)
{
  return ['posts' => 
PostRes::collection($this->whenLoaded('posts')),
        ];
    }

PostRes:
public function toArray($request)
{
return ['user' => 
new UserRes($this->whenLoaded('user'))
];
    }

Comment::resolveRelationUsing(
'user', function ($cM) {
    return $cM->belongsTo(
                 App\User::class, 'user_id','id');
});

App\User::resolveRelationUsing('posts', function ($uM) {
    return $uM->hasMany(Posts::class, 'user_id','id');
});

 $com=Comment::with(['user.posts'])->find(1);

// показывает связь posts
// dd($com->toArray()); 

return new CommentRes($com);
// не показывает связь posts