关于 golang mongo model 的映射问题

使用的库是 https://github.com/mongodb/mongo-go-driver

model 定义

type MUser struct {
	Id        bson.ObjectId `bson:"_id,omitempty"`
	CreateAt  int64         `bson: "createtime"`
	IsRemoved int           `bson: "hasremoved"`
	// 登录名
	Name string `bson: "name"`
	// 昵称
	Nick string `bson: "nick"`
	// 密码
	PassWord string `bson: "password"`
	Role     string `bson: "role"`
	Status   int    `bson: "status"`
}

为什么插入的数据是这个样子的？

{
    "_id" : ObjectId("5f747b15dedcfdbb4c4ab599"),
    "createat" : NumberLong(1601469205),
    "isremoved" : 0,
    "name" : "sample1",
    "nick" : "nick",
    "password" : "5f4dcc3b5aa765d61d8327deb882cf99",
    "role" : "dev",
    "status" : 0
}

请大家指点一下？