title: Accessing serializer instances in nested serializer's field
body: <p>Is there a way inside of a field in a nested serializer, to access the parent serializer's instance. Specifically the instance being accessed in a list view especially where the top most serializer may have multiple instances. I would like to pass this as context, but the context is passed by the view.</p>
  
  <p>When inside the field's to_representation, on a list for instance, I can access the list of instances on the parent in list view, but i'm not sure which one is the current instance being processed.</p>