schemas
    message.ts
    user.ts

messageSchema.post("remove", function(message) {
   User.findById(message.user, function (err, user) {
      user.messages.pull(message);
      user.save();
   });
}

import { Schema } from "mongoose";

const User = require('./user');

export const messageSchema: Schema = new Schema({
    createdAt: Date,
    content: {type: String, required: true},
    user: {type: Schema.Types.ObjectId, ref: 'User'}
});

messageSchema.pre("save", function(next) {
    if (!this.createdAt) {
        this.createdAt = new Date();
    }
    next();
});

messageSchema.post("remove", function(message) {
    User.findById(message.user, function (err, user) {
        user.messages.pull(message);
        user.save();
    });
});

import { Schema } from "mongoose";

const mongooseUniqueValidator = require('mongoose-unique-validator');

export const userSchema: Schema = new Schema({
    createdAt: Date,
    firstName: {type: String, required: true},
    lastName: {type: String, required: true},
    password: {type: String, required: true},
    email: {type: String, required: true, unique: true},
    messages: [{type: Schema.Types.ObjectId, ref: 'Message'}]
});

userSchema.plugin(mongooseUniqueValidator);

userSchema.pre("save", function(next) {
    if (!this.createdAt) {
        this.createdAt = new Date();
    }
    next();
});