title: Providing IDs for batch requesting
body: <p>I am designing an API and I want to allow the clients to be able to create several resources at once.  A client can POST an Order that has several Items.</p>
  
  <p>One approach would be to allow the client give unique ids at the Order level and for each of the items levels, and me storing those (as well as our internal ones), but that would allow the client to request based on its ids. Example of request:</p>
  
  <pre><code>{
    order_ref: 'XXX',
    items: [
    { item_ref: 'xx', quantity: 5 },
    { item_ref: 'yy', quantity: 5 },
    ]
  }
  </code></pre>
  
  <p>The other approach is to return an id for the order and for each of the items, but that seems not escalable as an order can have a lot of items so the request could even timeout.  It would also mean that the return would need to be in order so they can match the returned ids with what they requested.</p>
  
  <p>What is the approach that one should be taking?</p>