title: django-rest-framework how to make model serializer fields required
body: <p>I have a model that I'm filling out step by step, it means I'm making a form wizard.</p>
  
  <p>Because of that most fields in this model are required but have <code>null=True, blank=True</code> to avoid raising not null errors when submitting part of the data.</p>
  
  <p>I'm working with Angular.js and django-rest-framework and what I need is to tell the api that x and y fields should be required and it needs to return a validation error if they're empty.</p>