how to add another schema _id into another schema in nodejs code example

Example: mongoose reference another model

var mongoose = require('mongoose')
  , Schema = mongoose.Schema

var eventSchema = Schema({
    title     : String,
    location  : String,
    startDate : Date,
    endDate   : Date
});

var personSchema = Schema({
    firstname: String,
    lastname: String,
    email: String,
    gender: {type: String, enum: ["Male", "Female"]}
    dob: Date,
    city: String,
    interests: [interestsSchema],
    eventsAttended: [{ type: Schema.Types.ObjectId, ref: 'Event' }]
});

var Event  = mongoose.model('Event', eventSchema);
var Person = mongoose.model('Person', personSchema);