This is my model:

```
class Post(models.Model):
    owner = models.ForeignKey(User, related_name="%(app_label)s%(class)s_set")
    usersVoted = models.ManyToManyField(User, blank=True)
    post = models.CharField(max_length=400)

```

and this is my serializer:

```
class PostSerializer(serializers.ModelSerializer):

    class Meta:
        model = Post
        fields = ('owner', 'usersVoted', 'post')
        read_only_fields = ('owner', 'usersVoted')

    def create(self, validated_data):
        drop = Drop(
                owner =  User.objects.get(username='TestUser'),
                post = validated_data['post'],
        )

```

Owner has a ForeignKey to the default Django User model. Currently when I serialize a post, owner is the `pk` value of the user. How do I make owner the username of the user instead?