title: field must be unique on OneToOne field in Django DRF
body: <p>I'm using <code>Django 2.x</code> and <strong>DRF</strong>.</p>
  
  <p>I have a model to save rating for each entity</p>
  
  <pre><code>class UserRideRating(models.Model):
      user_ride = models.OneToOneField(UserRide, on_delete=models.CASCADE, related_name='user_ride_rating')
      rating = models.PositiveIntegerField(
          validators=[validate_rating_range]
      )
  </code></pre>
  
  <p>and <strong>serializers.py</strong></p>
  
  <pre><code>class UserRideRatingSerializer(serializers.ModelSerializer):
      class Meta:
          model = UserRideRating
          fields = ('id', 'user_ride', 'rating')
  </code></pre>
  
  <p>I have a view to create rating object if not already exists or update the rating if already exists. Also, I want to check validation using default Serializer validation and thus my view is like</p>
  
  <pre><code>@api_view(['POST'])
  def rate(request):
      serializer = UserRideRatingSerializer(data=request.data)
  
      if serializer.is_valid(raise_exception=True):
          # create or update data
          return Response(serializer.data)
  </code></pre>
  
  <p>When on passing data from the postman, it gives an error as</p>
  
  <pre><code>{
      "user_ride": [
          "This field must be unique."
      ]
  }
  </code></pre>
  
  <p>How can I enable check for only valid data and not the uniqueness of the field? Although <code>user_ride</code> should be unique, it can pass in the request data.</p>