Order by relationship column

You can use withAggregate function to solve your problem

UserItems::withAggregate('item','type')
    ->where('user_id','=',$this->id)
    ->where('quantity','>',0)
    ->orderBy('item_type')
    ->get();

I know it's an old question, but you can still use an "orderByRaw" without a join.

$items = UserItems
        ::where('user_id','=',$this->id)
        ->where('quantity','>',0)
        ->orderByRaw('(SELECT type FROM items WHERE items.id = user_items.item_id)')
        ->get();  

join() worked fine thanks to @rypskar comment

$items = UserItems
        ::where('user_id','=',$this->id)
        ->where('quantity','>',0)
        ->join('items', 'items.id', '=', 'user_items.item_id')
        ->orderBy('items.type')
        ->select('user_items.*') //see PS:
        ->get();

PS: To avoid the id attribute (or any shared name attribute between the two tables) to overlap and resulting in the wrong value, you should specify the select limit with select('user_items.*').

Well, your eager loading is probably not building the query you're expecting, and you can check it by enabling the query log.

But I would probably just use a collection filter:

$items = UserItems::where('user_id','=',$this->id)
        ->where('quantity','>',0)
        ->get()
        ->sortBy(function($useritem, $key) {
          return $useritem->item->type;
        });