title: Linq - flatten and sort by parent
body: <p>Simple question, not sure what the best answer is although I suspect it involves Linq.</p>
  
  <p>I have a list of "order" objects, each of which has an Id and a collection of "orderItem" objects, each of which in turn has an Id. However, the orderItem objects don't contain a direct reference to the "order" ID.</p>
  
  <p>I need to get a list of all the "orderItem" objects of all the "orders" in the list, sorted first by orderItemID and then orderID. </p>
  
  <p>(If I had a reference to orderID in orderItemID it would be very easy but I don't, and it'd be ugly and inefficient to add one just to do this).</p>
  
  <p>I can easily get the list of all the orderItem objects by iteration, obviously, but then I can't do the sorting I need. Thoughts?</p>