title: Common custom validation for DRF and Django admin
body: <p>I have user table in which have to put unique email constrain on staff users and unique phone constrain on non staff user. Now I want to maintain this constrain in DRF API and in Django admin, how to achieve this without code duplication.     </p>
  
  <p>As I see from DRF 3.0 onwards, all validations are performed explicitly for serializer (If I have overwritten unique_validation or clean method to perform conditional validation, I have to write this custom validation separately for serializer as well) so how to write a custom validation that works both for DRF API and Django admin without code duplication ?</p>