title: Django Rest Framework - How to get field of foreign key of a foreign key
body: <p>Assume I have these models:</p>
  
  <pre><code>ModelA(models.Model):
      some_field_a = models.IntegerField()
  
  ModelB(models.Model):
      some_field_b = models.IntegerField()
      a = models.ForeignKey(ModelA)
  
  ModelC(models.Model):
      some_field_c = models.IntegerField()
      b = models.ForeignKey(ModelB)
  </code></pre>
  
  <p>Then in my serializer for ModelC, I have this to access the field <em>some_field_b</em> from ModelB:</p>
  
  <pre><code>class ModelCSerializer(serializers.ModelSerializer):
      field_b = serializers.SlugRelatedField(
          read_only=True,
          source='some_field_b',
          slug_field='some_field_b')
  
      field_a = ???
  
      class Meta:
          model = ModelC
          fields = ('some_field_c', 'field_b', 'field_a')
  </code></pre>
  
  <p>But what should I add to <em>field_a</em> to retrieve the value of <em>some_field_a</em>, in the same way I'm retrieving the value from <em>some_field_b</em>?</p>