title: $firebaseArray join with other table
body: <p>I have a Firebase table named Orders: </p>
  
  <pre><code>{productID: xyz, 
   userID: abc, 
   quantity: 100}
  </code></pre>
  
  <p>There is another table Products containing product details. When showing a user's orders, I need to show some product details along with each order. How can I use <code>$firebaseArray</code> for this purpose?</p>
  
  <p><code>$firebaseArray</code> seems can only go with a table. Here, I'd need to query each ordered product's detail and add the detail to each order.  Is the <code>$extend</code> the way to go?  If so, any good example would be very appreciated.</p>