title: Instance for nested serializer is not looked up
body: <p>Using Django Rest Framework 3.1, I have a case as follows:</p>
  
  <pre><code># models.py
  
  class OrganizationUser(models.Model):
      user = models.ForeignKey(USER_MODEL)
      organization = models.ForeignKey(ORGANIZATION_MODEL)
  
  # serializers.py
  
  class UserSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = get_user_model()
  
  class OrganizationUserSerializer(serializers.ModelSerializer):
  
      user = UserSerializer()
  
      class Meta:
          model = OrganizationUser
  </code></pre>
  
  <p>I've simplified some of the above code to isolate the issue.</p>
  
  <p>The problem I'm having is that when I try to update an OrganizationUser through a viewset, I'm getting a validation error:</p>
  
  <pre><code>{"user":{"username":["This field must be unique."]}}
  </code></pre>
  
  <p>In debugging, I found that the error is raised by UniqueValidator, and is because the user instance is not set on the username field's parent (i.e., UserSerializer).</p>
  
  <p>So I added the followed to_internal_value implementation to OrganizationUserSerializer, and then it worked correctly:</p>
  
  <pre><code>def to_internal_value(self, data):
  
      if self.instance:
  
          # set user instance on its serializer
          user = self.instance.user
          self.fields['user'].instance = user
  
          # make sure correct user is specified in request data
          try:
              user_id = self.initial_data['user']['id']
          except KeyError:
              raise fw_serializers.ValidationError('User must be specified')
  
          if user_id != user.id:
              raise fw_serializers.ValidationError('Invalid user specified')
  
      return super(OrganizationUserSerializer, self).to_internal_value(data)
  </code></pre>
  
  <p>But I thought that the REST Framework would have done this automatically? Is that not the case? Is the application responsible to look up any nested serializer instances?</p>