title: Django Rest Framework PrimaryKeyRelatedField clearing if not set
body: <p>I have a serializer that inherits from <code>ModelSerializer</code> that looks something like this:</p>
  
  <pre><code>class MySerializer(serializers.ModelSerializer)
      set_favourite_books = serializers.PrimaryKeyRelatedField(source="favourite_books", many=True, required=False, write_only=True)
      set_favourite_movies = serializers.PrimaryKeyRelatedField(source="favourite_movies", many=True, required=False, write_only=True)
  
      class Meta:
          model = models.User
          fields = ('set_favourite_books', 'set_favourite_movies')
  </code></pre>
  
  <p>If I POST this it will work fine:</p>
  
  <pre><code>{ 
      'set_favourite_books': [1, 2, 3]
      'set_favourite_movies': [1, 2, 3]
  }
  </code></pre>
  
  <p>However if I try to only update one of the fields (not posting the other field entirely) like below:</p>
  
  <pre><code>{ 
      'set_favourite_books': [1, 2, 3]
  }
  </code></pre>
  
  <p>It will set the books, but clear the movies.</p>
  
  <p><strong>How can I get it to maintain the movies if the <code>set_favourite_movies</code> field is not submitted at all?</strong></p>