title: django-rest-framework: serializer from DATA don't update model ID
body: <p>I've a simple serializer like this:</p>
  
  <pre><code>class CategoryCreationSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = Category
  </code></pre>
  
  <p>Category has an id field (Django primary key) and I don't get why the Serializer is not updating it.</p>
  
  <p>Scenario: I've a BlogPost with a given category. I'm updating the BlogPost with a different Category (already created on the database). So the POST request will have all the BlogPost information with the new Category JSON object with the updated id:</p>
  
  <pre><code>{
      "id": 1,
      "title": "My first blog post",
      "category": {
          "id": 3,
          "name": "Techology"
      }
  }
  </code></pre>
  
  <p>The problem is that when I'm doing this:</p>
  
  <pre><code>category_data = data.get('category')
      if category_data is not None:
          category_serializer = CategoryCreationSerializer(data=category_data)
          if category_serializer.is_valid():
              blog_post.category = category_serializer.object
  </code></pre>
  
  <p>inside category title will be updated but the id field will be NONE.
  Can you explain me why? </p>
  
  <p>Cheers, Emanuele.</p>