title: Django REST Framework : "This field is required." with required=False and unique_together
body: <p>I want to save a simple model with Django REST Framework. The only requirement is that <code>UserVote.created_by</code> is set automatically within the <code>perform_create()</code> method. This fails with this exception:</p>
  
  <pre><code>{
      "created_by": [
          "This field is required."
      ]
  }
  </code></pre>
  
  <p>I guess it is because of the unique_together index.</p>
  
  <p>models.py:</p>
  
  <pre><code>class UserVote(models.Model):
      created_by = models.ForeignKey(User, related_name='uservotes')
      rating = models.ForeignKey(Rating)
  
      class Meta:
          unique_together = ('created_by', 'rating')
  </code></pre>
  
  <p>serializers.py</p>
  
  <pre><code>class UserVoteSerializer(serializers.ModelSerializer):
      id = serializers.IntegerField(read_only=True)
      created_by = UserSerializer(read_only=True)
  
      class Meta:
          model = UserVote
          fields = ('id', 'rating', 'created_by')
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>class UserVoteViewSet(viewsets.ModelViewSet):
      queryset = UserVote.objects.all()
      serializer_class = UserVoteSerializer
      permission_classes = (IsCreatedByOrReadOnly, )
  
      def perform_create(self, serializer):
          serializer.save(created_by=self.request.user)
  </code></pre>
  
  <p>How can I save my model in DRF without having the user to supply <code>created_by</code> and instead set this field automatically in code?</p>
  
  <p>Thanks in advance!</p>