title: one_to_many association through a join table
body: <p>I'm trying to do a one_to_many association through a join table.</p>
  
  <p>I have <code>Orders</code> with a <code>cart_id</code> field pointing to a <code>Cart</code>, which has no relevant keys. In the <code>CartItem</code> table there's a <code>cart_id</code> pointing to the cart. Poring through the documentation, I see how to implement the case where the <code>Cart</code> has a <code>order_id</code> column, the <code>many_to_many</code> association but not for the other way around. <code>many_to_many</code> seems to expect the left key to be in the join table. And <code>one_to_many</code> seems to not allow the use of a join table at all. Or maybe I'm just missing something?</p>