var clientSchema = new Schema({
    phone: String,
    email: String
    },
);

var menuSchema = new Schema({
    itemName: String,
    itemPrice: Number,
});


var transactionSchema = new Schema ({
    createdBy: { type: Schema.ObjectId, ref: 'Client'},
    items: [{ type: Schema.ObjectId, ref: 'Menu' }],
});

var Menu = mongoose.model('Menu', menuSchema);
var Client = mongoose.model('Client', clientSchema);
var Transaction = mongoose.model('Transaction', transactionSchema);

{
    "_id": "5a0bde94f4434c0a604341d2",
    "createdBy": {
        "_id": "5a0a8a3f9c348f0998ba8c2c",
        "phone": "1234567890",
        "email": "some@thing.com"
    },
    "__v": 0,
    "items": [{ Many Menu objects }]
}

{
    "_id": "5a0bde94f4434c0a604341d2",
    "createdBy": "5a0a8a3f9c348f0998ba8c2c",
    "__v": 0,
    "items": [Array of ObjectIds]
}

var transactionSchema = new Schema ({
    createdBy: { type: Schema.ObjectId, ref: 'Client'},
    items: [menuSchema], // Sub Doc
});

var transactionSchema = new Schema ({
    createdBy: { type: Schema.ObjectId, ref: 'Client'},
    items: [{
      name: {type: String},
      price: {type: Number}
    }]
});