title: Django Rest Framework, ModelSerializers and custom fields validation
body: <p>Using latest version of Django and DRF.</p>
  
  <p>I have a rather complex requirement I can't find a solution for. I'll try to simplify it. </p>
  
  <p>Let's say I have a model that has two fields. <code>field_a</code> and <code>field_b</code></p>
  
  <p>I have a <code>ModelSerializer</code> for it. I <code>POST</code> a request with its fields. The fields get validated with the model and then against my two serializer functions <code>validate_field_a</code> and <code>validate_field_b</code>. All is well.</p>
  
  <p>Now I'd like my <code>POST</code> request to include a third field that is not a member of that model. let's call it <code>field_c</code>. I have a custom <code>def create(self, validated_data):</code> in my serializer which saves everything to the database.</p>
  
  <p>with regards to <code>field_c</code> I would like to:</p>
  
  <ol>
  <li>Custom Validate it. just like I do with the other two fields.</li>
  <li>Require that it is mandatory for the whole request to succeed and if it's not, issue a "Field is required" error just like if I forgot to POST one of my required model fields. </li>
  <li>Have the chance to take <code>field_c</code> and save it onto a totally different unrelated Model's row in the db. </li>
  </ol>
  
  <p>I can't seem to get around that. If I add <code>field_c</code> to the <code>fields</code> meta - it throws an exception saying justifiably that <code>field_c</code> is not in my model. If I don't include it in fields, the <code>validate_field_c</code> which I really want to put there doesn't even get called. </p>
  
  <p>What can I do? </p>