如何在mongoDB中设置slug

我试图在MongoDB中创建一组用户待办事项,但我无法做到。

const mongoose = require('mongoose');
mongoose.Promise = global.Promise;
const slug = require('slugs');
const userSchema = new mongoose.Schema({
    name: {
        type: String,
        trim: true,
        required: 'Please Enter your Name',
    },
    bio: String,
    todos: [
        {
            task: {
                type: String,
                trim: true,
                required: 'Please Enter your Task',
            },
            slug: String,
            dueDate: Date,
            dueTime: String,
        },
    ],
});

userSchema.pre('save', function (next) {
    if (!this.isModified(this.todos.$.task)) return next();
    this.todos.$.slug = slug(this.todos.$.task);
    next();
});

module.exports = mongoose.model('User', userSchema);

据我所知,这就是你在mongoDB中创建slug的方法。但当我创造 todos 它并没有拯救 slug 在数据库中,其他一切都正常工作。