class ODM {

  constructor(document) {
    // get class name
    const name = this.constructor.name;

    // add unique id
    if (!document._id) document._id = Math.random().toString();

    // create document
    if (!db[name]) db[name] = {};
    db[name][document._id] = document;

    // define accessors
    const configuration = {};
    Object.keys(document).forEach((prop) => {
      configuration[prop] = {
        get() {
          const value = db[name][document._id][prop];
          // return an instance or a value
          return value.indexOf('@') !== -1 ? instances[value.replace('@', '')] : value;
        },
        set(value) {
          if (classes[value.constructor.name]) {
            // store the id of the instance
            db[name][document._id][prop] = value._id;
          } else {
            db[name][document._id][prop] = value;
          }
        }
      };
    });

    // set accessors
    Object.defineProperties(this, configuration);

    // add it to the list of instances
    instances[document._id] = this;
  }
}