to_field not working in ForeignKey relationship in django -

i trying link comments model username field of auth user model.
here code

from django.contrib.auth.models import user django.db import models urllib.parse import urlparse   class posts(models.model):     title = models.charfield(max_length=200)     url = models.urlfield()     points = models.integerfield(default=0)     moderator = models.foreignkey(user, related_name="moderated_stories")     voters = models.manytomanyfield(user, related_name="liked_stories", blank=true)     created_at = models.datetimefield(auto_now_add=true)     updated_at = models.datetimefield(auto_now=true)      @property     def domain(self):             return urlparse(self.url).netloc      def __str__(self):             return self.title + " " + self.moderator.username      class meta:             verbose_name_plural = "stories"   class comments(models.model):      comment = models.textfield()     created_at = models.datetimefield(auto_now_add=true)     user = models.foreignkey(user, related_name="commented_on", to_field="username")     commented_on = models.foreignkey(posts, related_name="comments")      def __str__(self):             return self.comment + " " + self.user      class meta:             verbose_name_plural = "comments" 

but on executing queries gives me following error:

>>> post = posts.objects.get(id=2) >>> post.comments.all() django.contrib.auth.models.doesnotexist: user matching query not exist. 

but when execute query serialize data works

>>> serializers.serialize('json', post.comments.all()) '[{"model": "news.comments", "pk": 1, "fields": {"comment": "thanks sharing link :)", "commented_on": 2, "created_at": "2015-09-30t06:01:40.201z", "user ": 1}}, {"model": "news.comments", "pk": 2, "fields": {"comment": "this looking for...!", "commented_on": 2, "created_at": "2015-09-30t06:02:17.4 97z", "user": 4}}, {"model": "news.comments", "pk": 3, "fields": {"comment": "th anks sharing !", "commented_on": 2, "created_at": "2015-09-30t06:02:43.971z" , "user": 3}}]' 

but serialized data still contains pk of user , not username. wrong code?