Laravel OrderBy relationship count

This works for me in Laravel 5.3, using your example:

Hackathon::withCount('participants')->orderBy('participants_count', 'desc')->paginate(10); 

This way it is ordered on the query and the pagination works nicely.

Another approach can be by using withCount() method.

Hackathon::withCount('participants')
        ->orderBy('participants_count', 'desc')
        ->paginate(50);

Ref: https://laravel.com/docs/5.5/eloquent-relationships#querying-relations

I had similar issue and using sortBy() is not suitable because of pagination, exactly as Sabrina Gelbart commented in previous solution. So I used db raw, here's simplified query:

Tag::select( 
array(
    '*',
    DB::raw('(SELECT count(*) FROM link_tag WHERE tag_id = id) as count_links')) 
)->with('links')->orderBy('count_links','desc')->paginate(5);   

Edit: If using Laravel 5.2 or greater, use kJamesy's answer. It will likely perform a bit better because it's not going to need to load up all the participants and hackathons into memory, just the paginated hackathons and the count of participants for those hackathons.

You should be able to use the Collection's sortBy() and count() methods to do this fairly easily.

$hackathons = Hackathon::with('participants')->get()->sortBy(function($hackathon)
{
    return $hackathon->participants->count();
});