title: Relay RANGE_ADD queryConfig on SearchType
body: <p>I'm trying to understand what config should be used for <code>RANGE_ADD</code> mutation if <code>parent</code> of the new item is something like <code>SearchType</code> which doesn't have an ID attribute.
  What should be used as <code>parentId</code> in this case or how this mutation should be configured? If I don't specify <code>parentID</code> then newly created item won't be added to the Relay store.</p>
  
  <p>I've read lots of issue in Relay repo but it's still unclear to me how to handle this mutation properly.</p>