Node.js 在mongodb中将_id显示为子文档的第一个元素

我有一个带有子文档的文档，每个子文档都有一个id。文档的id元素总是第一个。但在子文档中，它不是第一个元素

{
    "_id" : ObjectId("5848140981adc51dac264dad"),
    "name" : "alpha",
    "workouts" : [
            {
                    "date" : "07-12-2016",
                    "timestamp" : 1481049000000,
                    "day_code" : 1,
                    "goal" : "weight_loss",
                    "_id" : ObjectId("5848144281adc51dac264dae"),
                    "workout" : [
                            {
                                    "exercise_name" : "skater_hops",
                                    "duration" : 30,
                                    "recommended_reps" : null,
                                    "recommended_weight" : null,
                                    "_id" : ObjectId("5848144281adc51dac264daf"),
                                    "details" : [ ]
                            }

虽然这对实际目的没有影响，但只是为了便于阅读

模式：

var WorkoutSchema = new Schema({
   date : {type : String, required : true},
   timestamp : {type : Number, required : true},
   day_code : {type : Number, required : true},
   goal : {type : String, required : true},
   workout : [WorkoutExerciseSchema]
});

var Workout = mongoose.model('Workout', WorkoutSchema, 'user');

var UserSchema = new Schema({
   name : {type : String, required : true},
   workouts : [WorkoutSchema]
});