title: django-rest-framework 3.0 create or update in nested serializer
body: <p>With <strong>django-rest-framework 3.0</strong> and having these simple models:</p>
  
  <pre><code>class Book(models.Model):
      title = models.CharField(max_length=50)
  
  
  class Page(models.Model):
      book = models.ForeignKey(Books, related_name='related_book')
      text = models.CharField(max_length=500)
  </code></pre>
  
  <p>And given this JSON request:</p>
  
  <pre><code>{
     "book_id":1,
     "pages":[
        {
           "page_id":2,
           "text":"loremipsum"
        },
        {
           "page_id":4,
           "text":"loremipsum"
        }
     ]
  }
  </code></pre>
  
  <p>How can I write a nested serializer to process this JSON and for each <code>page</code> for the given <code>book</code> either create a new page or update if it exists.</p>
  
  <pre><code>class RequestSerializer(serializers.Serializer):
      book_id = serializers.IntegerField()
      page = PageSerializer(many=True)
  
  
  class PageSerializer(serializers.ModelSerializer):
      class Meta:
          model = Page
  </code></pre>
  
  <p>I know that instantiating the serializer with an <code>instance</code> will update the current one but how should I use it inside the <code>create</code> method of nested serializer? </p>