title: How to Serialize generic foreign key In DRF
body: <p>I have model with generic foreign key and I want to serialize that model.</p>
  
  <p><code>model.py</code>:</p>
  
  <pre><code>class AddressType(models.Model):
      content_type = models.ForeignKey(ContentType)
      object_id = models.PositiveIntegerField()
      content_object = GenericForeignKey('content_type','object_id')
  
      def __unicode__(self):
          return u'%s' % str(self.content_type)
  
  class AddressBook(TimeStampedModel):
      class Meta:
          db_table = 'address_book'
  
      uuid = UUIDField(auto=True)
      address_tag = models.CharField(null=True, blank=True, max_length=20)
  
      # should be a generic foreign key
      address_object_type = GenericRelation(AddressType)
      address1 = models.CharField(
          verbose_name='Address1',
          max_length=200,
      )
      address2 = models.CharField(
          verbose_name='Address2',
          max_length=200,
      )
  </code></pre>
  
  <p><code>serializer.py</code>:</p>
  
  <pre><code>class AddressBookSerializer(serializers.ModelSerializer):
      class Meta:
          model = AddressBook
          fields = ('id','uuid','address_tag','address_object_type','address1','address2')
  </code></pre>
  
  <p>How can I serialize JSON on above model?</p>