title: Eloquent pivot : how to do select columns in Laravel 5.1
body: <p>I have two models <code>Bill</code> &amp; <code>Order</code>,
  I can get <code>Orders</code> records via :</p>
  
  <pre><code>    $orders = $bill-&gt;orders;
  </code></pre>
  
  <p>But I want to use select like this :</p>
  
  <pre><code> $orders = $bill-&gt;orders-&gt;select([
                              'orders.*',                            
                              \DB::raw('sum(orders.quantity) as qty'),
                              \DB::raw('sum(orders.price) as  total')                            
                              ]) 
                              -&gt;groupBy('article_id');
  </code></pre>
  
  <p>I get this error :</p>
  
  <pre><code>Method select does not exist.
  </code></pre>
  
  <p>Please any one may help me, thanks !</p>