volume_create用于外键列表，提供意外输出

Question

bulk_create给出意想不到的输出，这里有什么问题...... ????

class User(models.Model):
    first_name =  models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)

class MessagePost(models.Model):
    send_to = models.ForeignKey(User)
    media_def = models.TextField(blank=True)
    message = models.TextField()

#userList fetched from db
userList = [User(first_name='a',last_name='b'),User(first_name='x',last_name='y')]

MessagePost.objects.bulk_create([MessagePost(send_to=u , media_def='media/images/1.png' , message='hai everyone') for u in userList ])

在使用bulk_create时，它会出现以下错误的 输出 ：

TypeError : expected string or buffer