在django admin中过滤下拉列表值

class Foo(models.Model):
    title = models.TextField()
    userid = models.IntegerField()
    image = models.CharField(max_length=100)
    def __unicode__(self):
       return self.title

class Bar(models.Model):
    foo = models.ForeignKey(Foo, related_name='Foo_picks', unique=True)
    added_on = models.DateTimeField(auto_now_add=True)
class Foo(models.Model):
    title = models.Tex