MGO return bson field instead of json field

The bson name is used when performing pipe in mgo.
Struct :

type Training struct {
	Id                  bson.ObjectId   `json:"id" bson:"_id"`
	Name                string          `json:"name" bson:"name"`
	Description         string          `json:"description"`
	Level               *TrainingLevel  `json:"level"`
	Preworks            []bson.ObjectId `json:"preworks"`
	PrePostTests        []bson.ObjectId `json:"preposttests" bson:"preposttests"`
	TrainingEvaluations []bson.ObjectId `json:"training_evaluations" bson:"training_evaluations"`
	TrainerEvaluations  []bson.ObjectId `json:"trainer_evaluations" bson:"trainer_evaluations"`
	AppCompanyId        bson.ObjectId   `json:"app_company_id" bson:"app_company_id"`
	Company             *Company        `json:"company"`
}

Function :

func (this *TrainingClass) GetAllTraining() (interface{}, error) {
	if !this.tokenInfo.IsAllowed(this.c) {
		return nil, tlib.NewTError(common.Error_NoAccess, "You don't have the right!")
	}
	sess, db := GetDB()
	defer sess.Close()

	pipeline := []bson.M{
		{"$match": bson.M{
			"app_company_id": this.tokenInfo.AppCompanyId}},
		{"$lookup": bson.M{
			"from":         "trainingbatch",
			"localField":   "_id",
			"foreignField": "training._id",
			"as":           "trainingbatches"}},
	}

	resp := []bson.M{}
	db.C(common.C_TRAINING).Pipe(pipeline).All(&resp)

	return bson.M{"data": resp}, nil
}

Json result :

{
  "data": [
    {
      "_id": "5995a749dbcfbe4e8cc31378",
      "app_company_id": "58b24756e65bd121f6b1a923",
      "description": "Description First Training",
      "name": "First Training",
      "trainingbatches": [
        {
          "_id": "5995a74adbcfbe4e8cc31379",
          "app_company_id": "58b24756e65bd121f6b1a923",
          "company": {
            "_id": "58b24756e65bd121f6b1a923",
            "address": "",
            "app_company_id": "58b24756e65bd121f6b1a923",
            "fullname": "",
            "name": "Tandem",
            "phone": ""
          },
        }
      ]
    }
  ]
}

As you can see field _id is generated instead of id. That's not happen if I use find or findId. Is there any way to keep using json field no matter what's the query?

The way you're reading the result, it has no idea what the JSON field names are. In order for it to use those tags, it must actually deserialize into the struct where the tags have been specified. When you do:

    resp := []bson.M{}
    db.C(common.C_TRAINING).Pipe(pipeline).All(&resp)

You're explicitly telling mgo to return BSON results. The object you pass in (a slice of bson.M) has no json tags on it. In order to control the serialization to JSON, you must pass a struct with the JSON tags specified to All:

    resp := []Training
    db.C(common.C_TRAINING).Pipe(pipeline).All(&resp)