title: Django Rest Framework Serializer - return related field
body: <p>I have a model with a one-to-one relationship with a main model:</p>
  <pre><code>class User(models.Model):
      id = models.BigIntegerField(primary_key=True)
      username = models.CharField(max_length=100, blank=True)
  
  class AggregatedStats(models.Model):
      user_id = models.ForeignKey('User', on_delete=models.DO_NOTHING, unique=True)
      followers_30d = models.BigIntegerField(blank=True)
  </code></pre>
  <p>I have written the following serializers:</p>
  <pre><code>class UserSerializer(serializers.HyperlinkedModelSerializer):
  
      class Meta:
          model = User
          fields = ['id', 'username', 'followers']
  
  class AggregatedStatsSerializer(serializers.HyperlinkedModelSerializer):
  
      username = UserSerializer(source='user.username')
  
      class Meta:
          model = AggregatedStats
          fields = ['followers_30d', 'username']
  </code></pre>
  <p>I am trying to return the <code>username</code> from the <code>User</code> model, but whatever I try to get it, the best I can do is get the hyperlinked related field from user, but not the actual &quot;username&quot; attribute. How would you return this?</p>