ustk-todolist/models/Todo.js

const mongoose = require('mongoose');

const { Schema } = mongoose;

const TodoSchema = Schema({
  text: {
    type: String,
    required: true,
    minLength: 1,
    maxLength: 300,
    trim: true,
  },
  list: { type: Schema.Types.ObjectId, ref: 'TodoList', required: true },
  user: { type: Schema.Types.ObjectId, ref: 'User', required: true },
  completed: { type: Boolean, default: false },
});

TodoSchema.pre('save', async function() {
  if (this.isNew) {
    const user = await this.model('User').findById(this.user);
    user.todos.push(this._id);
    await user.save();

    const list = await this.model('TodoList').findById(this.list);
    list.todos.push(this._id);
    await list.save();
  }
});

TodoSchema.pre('remove', async function() {
  const user = await this.model('User').findById(this.user);
  user.todos.splice(user.todos.indexOf(this._id), 1);
  await user.save();

  const list = await this.model('TodoList').findById(this.list);
  list.todos.splice(list.todos.indexOf(this._id), 1);
  await list.save();
});

TodoSchema.methods.toJson = function() {
  return {
    id: this._id.toString(),
    text: this.text,
    list: this.list.toString(),
    user: this.user.toString(),
    completed: this.completed,
  };
};

mongoose.model('Todo', TodoSchema);