title: django rest framework: set field-level error from serializer validate() method
body: <p>I have a serializer that validates fields based on the values of other fields, In the error response I would like to show each field error as a field error as opposed to showing everything under "non_field_errors" which is what would happen if I were to raise a ValidationError in the object-level validate method. Below is an illustration of what I'm trying to achieve:</p>
  
  <pre><code>MySerializer(ModelSerializer):
      ...
      def validate(self, data):
          field_val1 = data['field_val1']
          field_val2 = data['field_val2']
          if not self._is_field_valid(field_val1, field_val2):
              # The below line is how I would do what I want with Django
              # Forms, however, it's not valid in DRF
              self._errors['field_val1'] = 'this field is not valid'
  </code></pre>
  
  <p>The desired error response is:</p>
  
  <pre><code>{'field_val1': ['this field is not valid']}
  </code></pre>