reference.js

// Books Schema
var bookSchema = new Schema({
    name: String,
    postedBy: {type: mongoose.Schema.Types.ObjectId, ref: 'User'},
    dateCreated: Date,
});

// Registering Schema
var Book = mongoose.model('Books', bookSchema);

// User Schema
var userSchema = new Schema({
    fullName: String,
    age: Number,
    books: {type: mongoose.Schema.Types.ObjectId, ref: 'Books'
});

// Registering Schema
var User = mongoose.model('User', userSchema);
  
Book.findOne({_id: 123})
.populate('users')
.exec(function(err, post) {
    // code here
});