title: get_identity() of Django REST Framework 2 substitute in version 3?
body: <p>In DRF2 there was <a href="http://tomchristie.github.io/rest-framework-2-docs/api-guide/serializers#how-identity-is-determined-when-performing-bulk-updates" rel="nofollow">get_identity()</a> method, that can be overridden to customize matching serialized data with database objects.</p>
  
  <p>In the new version it's gone, but is there a way to do the same thing? </p>