MongoDB管道（Golang）仅在文档内部返回四个字段中的一个

I was trying to dump data received from a lookup using mgo/pkg package but when I receive data, dump it and return, I get this output:

 [
  {
    "_id": "",
    "timerId": "",
    "content": "task1",
    "creationDate": 0
  },
  {
    "_id": "",
    "timerId": "",
    "content": "task2",
    "creationDate": 0
  },
  {
    "_id": "",
    "timerId": "",
    "content": "task3",
    "creationDate": 0
  }
]

Printing with fmt:

[{ObjectIdHex("") ObjectIdHex("") task1 0} {ObjectIdHex("") ObjectIdHex("") task2 0} {ObjectIdHex("") ObjectIdHex("") task3 0}]

Code that is performing this action:

func (m *MongoDbRepository) GetTasksOnTheSameDateAsUserTimers(userId string)([]model.Task, error) {

    var tasks []model.Task
    // m.Db.Session.DB("project").C("tasks")
    pipe := m.Db.C("tasks").Pipe([]bson.M{
        // bson.M{
        //  "$lookup": bson.M{
        //      "from":         "times",
        //      "localField":   "timerId",
        //      "foreignField": "_id",
        //      "as":           "timers",
        //  },
        // },
        // bson.M{
        //  "$lookup": bson.M{
        //      "from":         "users",
        //      "localField":   "timers.userId",
        //      "foreignField": "_id",
        //      "as":           "userTasks",
        //  },
        // },
        // bson.M{
        //  "$project": bson.M{
        //      "timers": 0,
        //  },
        // },
        // bson.M{
        //  "$match": bson.M{ 
        //      "_id":bson.ObjectIdHex("5cb58c5c4d11610c20b52f1a"),
        //  },
        // },
    })

    err := pipe.All(&tasks)
    fmt.Println(tasks)
    return tasks, err
}

I have commented all inside pipe because I can't perform the lookup because of I can't get the result of the piping correctly. I know that is a dumping data error because if I change ending lines like this:

var tasksDump []interface{}
err := pipe.All(&tasksDump)
fmt.Println(tasksDump)

Then this is the output:

[map[_id:ObjectIdHex("5cb58c5c4d11610c20b52f1a") content:task1 creationDate:1555400713 timerId:ObjectIdHex("5cb5880891b3fb1e2830b743")] map[_id:ObjectIdHex("5cb58ccd4d11610c20b52f1b") content:task2 creationDate:1555400714 timerId:ObjectIdHex("5cb5880891b3fb1e2830b743")] map[_id:ObjectIdHex("5cb58d074d11610c20b52f1c") content:task3 creationDate:1555400700 timerId:ObjectIdHex("5cb5880891b3fb1e2830b743")]]

So data is arriving but when parse I have the issue. Why only content field is being parsed correctly?

Code of the struct where I am dumping data from lookup:

type Task struct {
    Id          bson.ObjectId       `json:"_id"             bson:"_id"`
    TimerId         bson.ObjectId       `json:"timerId"         bson:"timerId"`
    Content         string          `json:"content"         bson:"content"`
    CreationDate            int64           `json:"creationDate"     bson:"creationDate"`
}

MongoDB collection from where I'm receiving data:

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