title: Unique validation on nested serializer on Django Rest Framework
body: <p>I have a case like this, where you have a custom nested serializer relation with a unique field. Sample case:</p>
  
  <pre><code>class GenreSerializer(serializers.ModelSerializer):
  
      class Meta:
          fields = ('name',) #This field is unique
          model = Genre
  
  class BookSerializer(serializers.ModelSerializer):
  
      genre = GenreSerializer()
  
      class Meta:
          model = Book
          fields = ('name', 'genre')
  
      def create(self, validated_data):
          genre = validated_data.pop('genre')
          genre = Genre.objects.get(**genre)
          return Book.objects.create(genre=genre, **validated_data)
  </code></pre>
  
  <p><strong>The problem:</strong> When i try to save a json object like {"name":"The Prince", "genre": {"name": "History"}} DRF try to validate the genre object unique constraint and if <em>"History"</em> exists throw me an exception because a genre with name <em>"History"</em> must be unique, and that's true but i just trying to relate the object and not create together.</p>
  
  <p>Thank you a lot!!</p>