why _id is set to null for subdocuments in default in mongoose

Question

i need a _id for my subdocuments but it set default to null,but the field _id id generated automatically but the value set to null, i tried in many ways but is default value null is not changed.

method 1

const event_Schema = mongoose.Schema({
    bid: {
        type: String,
    },
    name: {
        type: String,
    },
    start_date: {
        type: String,
    },
    end_date: {
        type: String,
    },
    start_time: {
        type: String,
    },
    end_time: {
        type: String,
    },
    slot_duration: {
        type: String,
    },
    excuse: {
        type: String,
    },
    slots: [
        {
            date: { type: String },
            slots: [
                {
                    uid: {
                        type: String,
                    },
                    start_time: {
                        type: String,
                    },
                    end_time: {
                        type: String,
                    },
                    attachment: {
                        type: String,
                    },
                    description: {
                        type: String,
                    },
                    available: {
                        type: Boolean,
                    },
                    status: {
                        type: String,
                        //default: 'unbook',
                    },
                },
            ],
        },
    ],
})

Method 2

const slot_Schema = mongoose.Schema({
    uid: {
        type: String,
    },
    start_time: {
        type: String,
    },
    end_time: {
        type: String,
    },
    attachment: {
        type: String,
    },
    description: {
        type: String,
    },
    available: {
        type: Boolean,
    },
    status: {
        type: String,
    },
})
const slots_Schema = mongoose.Schema({
    date: { type: String },
    slots: [slot_Schema],
})

const event_Schema = mongoose.Schema({
    bid: {
        type: String,
    },
    name: {
        type: String,
    },
    start_date: {
        type: String,
    },
    end_date: {
        type: String,
    },
    start_time: {
        type: String,
    },
    end_time: {
        type: String,
    },
    slot_duration: {
        type: String,
    },
    excuse: {
        type: String,
    },
    slots: [slots_Schema],
})

is there is any way to resove this or i need to use any other 3rd party packages to generate _id

version: "mongoose": "^6.6.5",