title: Get results by finding an ID which is two joins away using Eloquent relationships
body: <p>I need to get rows from one table using an id which is two joins away.
  I know I can use <code>join('table_name')</code> but I am trying to use the model names rather than raw table names.</p>
  <p>I'm trying to select <code>shipping_shipment.*</code> by joining <code>order_item_join_shipping_shipment</code> then joining <code>order_item</code>, and filtering <code>where order_item.order_id = x</code></p>
  <p>I tried this in the ShippingShipment class, but I can't figure it out.</p>
  <pre><code>return $this-&gt;hasManyThrough(OrderItem::class, ShippingShipment::class, 'shipment_id', 'order_item_id', 'id', 'id');
  </code></pre>
  <p>There are many items in an order, and many shipments. I need to get the shipments.<br/>
  There can be more than one shipment per order - items come from various places.<br/>
  There can be more than one shipment per item - if something is returned and needs shipping again.</p>
  <p>The table I want to get rows from, shipping_shipment, is joined to order_item by a join table order_item_join_shipping_shipment. That join table has the order_item_id. I need then to join order_item table so that I can search for order_item.order_id</p>
  <p>Table <strong>order_item</strong> model OrderItem</p>
  <pre><code>+-----+---------------+
  | id  | order_id      |
  +-----+---------------+
  |  6  |     13464     |
  |  8  |     13464     |
  |  9  |     13464     |
  +-----+---------------+
  </code></pre>
  <p>Table <strong>order_item_join_shipping_shipment</strong> model OrderItemJoinShippingShipment</p>
  <pre><code>+-----+---------------+-------------+
  | id  | order_item_id | shipment_id |
  +-----+---------------+-------------+
  |  1  |       6       |     12      |
  |  1  |       9       |     12      | two items in one shipment
  |     |               |             |
  |  2  |       8       |     13      |
  |  3  |       8       |     14      | one item was returned so shipped again
  +-----+---------------+-------------+
  </code></pre>
  <p>Table <strong>shipping_shipment</strong> don't need describing except to say it has an id column.</p>
  <p>If I was to do it with MySQL it would look like this</p>
  <pre><code>SELECT ss.*, oiss.order_item_id FROM 
      order_item_join_shipping_shipment AS oiss
      INNER JOIN shipping_shipment AS ss ON (oiss.shipment_id = ss.id)
      INNER JOIN order_item AS oi ON (oiss.order_item_id = oi.id)
  WHERE oi.order_id = 13464
  </code></pre>