title: Creating new fields in serializer that access another model's attributes
body: <p>My API call to <code>api/business-review/3abe3a1e-199c-4a4b-9d3b-e7cb522d6bed/</code> currently returns the following:</p>
  
  <pre><code>[
      {
          "id": "3abe3a1e-199c-4a4b-9d3b-e7cb522d6bed",
          "date_time": "2016-05-31T19:18:24Z",
          "review": "Another quality job, Anna has a no fuss approach to his job and clearly takes pride in what he does. Will continue to use again and again.",
          "rating": "4.0",
          "person": "c1cc5684-1be1-4120-9d81-05aec29f352a",
          "employee": "ecdc1f99-138c-4f9f-9e1f-b959d59209aa",
          "service": "1dfa408f-d5bc-4eb2-96ae-e07e7999a01a",
      }
  ]
  </code></pre>
  
  <p>Now I want to create three new fields: </p>
  
  <ul>
  <li><p><code>person_name</code> - which grabs the first_name and last_name of the reviewer</p></li>
  <li><p><code>employee_name</code> - which grabs the first_name and last_name of the employee</p></li>
  <li><p><code>service_name</code> - which grabs the title of the service</p></li>
  </ul>
  
  <p>I've tried the following so far but it doesn't create any new variables:</p>
  
  <p><strong>serializers.py</strong></p>
  
  <pre><code>class ReviewSerializer(serializers.ModelSerializer):
      """
      Class to serialize Review objects
      """
      person_name = serializers.CharField(source='person.reviewer.first_name', read_only=True)
      employee_name = serializers.CharField(source='person.employer.first_name', read_only=True)
      service_name = serializers.CharField(source='service.title', read_only=True)
  
      class Meta:
          model = Review
          fields = '__all__'
          read_only_fields = 'id'
  </code></pre>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Review(models.Model):
      """
      Review model
      """
      id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
      date_time = models.DateTimeField(blank=True, null=True, default=None)
      person = models.ForeignKey(Person, null=True, default=None, related_name='reviewer')
      employee = models.ForeignKey(Person, null=True, default=None, related_name='employee')
      review = models.TextField(null=True, default=None)
      service = models.ForeignKey(Service, null=True, default=None)
      rating = models.DecimalField(max_digits=4, decimal_places=1)
  
  class Person(models.Model):
      """
      Person entity
      """
      id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
      first_name = models.CharField(max_length=255)
      last_name = models.CharField(max_length=255)
  
  class Service(models.Model):
      """
      Service model
      """
      id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
      title = models.CharField(max_length=255)
  </code></pre>