title: Django rest framework serializer with extra field
body: <p>I have a django app in which i am using Django Rest Framework!</p>
  
  <p>I have a model as follows:</p>
  
  <pre><code>class Author(models.Model):
      first_name = models.CharField(max_length=30)
      last_name = models.CharField(max_length=40)
      email = models.EmailField()
  
      def __unicode__(self):
          return u'%s %s' % (self.first_name, self.last_name)
  </code></pre>
  
  <p>I have the serializer defined as follows:</p>
  
  <pre><code>class AuthorSerializer(serializers.ModelSerializer):
      class Meta:
          model = Author
          fields = ('first_name', 'last_name', 'email')
  </code></pre>
  
  <p>In my view, i have a query as follows:</p>
  
  <pre><code>author_detail = Author.objects.extra(select={'middle_name':'''SELECT middle_name from MyTable where MyTable.first_name = first_name'''})
  </code></pre>
  
  <p>According to the query, i will be getting an extra field in my objects called <code>middle_name</code>. But i cannot use the serializer on the resulting object anymore because it will not bring the <code>middle_name</code> in the api. </p>
  
  <p>How do i modify my serializer to get the <code>middle_name</code> as well?</p>