Laravel merge excluding the primary object

My question is very basic. I'm aware that it was asked before. But those questions has the answers which used to work in Laravel 4 and I can't quite get it to work in Laravel 5.6.

I'm trying to merge these two:

$posts = Post::where('user_id', $user->user_id)->get();
$comments = Comment::where('user_id', $user->user_id)->get();

The method I tried is this:

$activities = $posts->merge($comments)->sortBy('created_at');

This gives me only $comments In previous questions this seem to work in most cases but not in mine.

Also, I know there is an option to do it like

$activities = array_merge($posts->toArray(), $comments->toArray());

But when I do that I'm no longer able to detect which model or table the object belongs to or can't use the relationships.

I'm open to any solution with that, but I would love the ones where I can still use my relationships.

Laravel 5.6

via Chebli Mohamed