将具有外键的模型序列化为Django Auth User

Question

如何将具有外键的模型序列化为Django auth用户，以便可以检索用户名？这是我通过Tastypie尝试过的：

def dehydrate(self, bundle):
    posts = Post.objects.annotate(num_poster = Count('likedby')).order_by('-num_poster')
specificpost = posts.get(id__exact=bundle.data['id'])
likedList = specificpost.likedby.all()[:7]
liked_data = serializers.serialize('python', likedList, fields=('user__username', 'userPic'), use_natural_keys=True)
liked_actual = [d['fields'] for d in liked_data]
    bundle.data['likepreview'] = liked_actual
    return bundle

在我得到的JSON中，我可以看到userPic字段，但没有用户名字段，为什么会这样？

 {
"meta": {
    "limit": 20,
    "next": null,
    "offset": 0,
    "previous": null,
    "total_count": 17
},
"objects": [{
    "id": "19",
    "liked": 0,
    "likepreview": [{
        "userPic": "http://s3.amazonaws.com/post/mike/profile/photo/mikeProfilePic.jpg"
    }