title: Doing an orderBy through relationship
body: <p>I've got a pretty normal table structure like this:</p>
  
  <pre><code> Order
  ,----------,
  |id        | 
  |order_num |
  `----------`
  
   OrderItem
  ,---------,
  |id       |
  |order_id |
  |item_id  |
  `---------`
  
   Item
  ,----------,
  |id        |
  |item_name |
  |price     |
  `----------`
  </code></pre>
  
  <p>A classic <code>Order</code> table, <code>Item</code> table, and adding items to an order via an <code>OrderItem</code> table. However, what I want to be able to do is get a list of <code>OrderItem</code> records that are ordered by <code>Item</code>'s item_name column from the Order model.</p>
  
  <pre><code>class Order extends Model 
  {
       public function order_items()
       {
           return $this-&gt;hasMany('App\OrderItem');   &lt;-- How can I get this ordered by the item name?
       }
  }
  </code></pre>
  
  <p>So it's sort of an <code>-&gt;orderThrough</code> (if such a thing existed). Is there a clean way to accomplish this?</p>