{"id":396,"date":"2015-09-18T23:25:16","date_gmt":"2015-09-18T21:25:16","guid":{"rendered":"http:\/\/joost.vunderink.net\/blog\/?p=396"},"modified":"2015-10-12T16:11:08","modified_gmt":"2015-10-12T14:11:08","slug":"combining-mongoose-and-q-in-node-js","status":"publish","type":"post","link":"https:\/\/joost.vunderink.net\/blog\/2015\/09\/18\/combining-mongoose-and-q-in-node-js\/","title":{"rendered":"Combining mongoose and Q in node.js"},"content":{"rendered":"

This post will teach you how to write promise-based<\/a> mongoose<\/a> code, using Kris Kowal’s Q library<\/a>.<\/p>\n

You can also find the code below in the `src\/mongoose-and-q` dir of my blog code repository<\/a>.<\/p>\n

We\u00a0have a mongo database with 3 collections: users, bicycles and cars. Bicycles and cars are owned by users. We\u00a0will write a script that shows the bicycles and cars owned by\u00a0rupert@example.com<\/span>\u00a0.<\/p>\n

We’ll look at multiple\u00a0versions of this script. This article\u00a0focuses on callbacks and promises, so we’ll be using a hardcoded email address and we’ll not be handling errors very well. Those aspects of the code are for a different article.<\/p>\n

The first version uses callbacks for the database calls.<\/p>\n

'use strict';\r\n\r\nvar mongoose = require('mongoose');\r\nvar Schema   = mongoose.Schema;\r\n\r\nvar UserSchema = new Schema({\r\n  email    : { type: String },\r\n  firstName: { type: String },\r\n});\r\nvar User = mongoose.model('User', UserSchema);\r\n\r\nvar CarSchema = new Schema({\r\n  ownerId       : { type: Schema.Types.ObjectId, ref: 'User' },\r\n  colour        : { type: String },\r\n  numberOfWheels: { type: Number },\r\n});\r\nvar Car = mongoose.model('Car', CarSchema);\r\n\r\nvar BicycleSchema = new Schema({\r\n  ownerId       : { type: Schema.Types.ObjectId, ref: 'User' },\r\n  colour        : { type: String },\r\n  numberOfWheels: { type: Number },\r\n});\r\nvar Bicycle = mongoose.model('Bicycle', BicycleSchema);\r\n\r\nmongoose.connect('mongodb:\/\/localhost\/blogtest');\r\n\r\nvar email = 'rupert@example.com';\r\n\r\nfunction getVehicles(email, cb) {\r\n  User.findOne({ email: email}, function(err, user) {\r\n    if (err) {\r\n      return cb(err);\r\n    }\r\n    Car.find({ ownerId: user._id }, function(err, cars) {\r\n      if (err) {\r\n        return cb(err);\r\n      }\r\n      Bicycle.find({ ownerId: user._id }, function(err, bicycles) {\r\n        if (err) {\r\n          return cb(err);\r\n        }\r\n        cb(null, {\r\n          cars: cars,\r\n          bicycles: bicycles\r\n        });\r\n      });\r\n    });\r\n  });\r\n}\r\n\r\ngetVehicles(email, function(err, vehicles) {\r\n  if (err) {\r\n    console.error('Something went wrong: ' + err);\r\n  }\r\n  else {\r\n    console.info(vehicles);\r\n  }\r\n  mongoose.disconnect();\r\n});\r\n<\/pre>\n
Two things stand out in this code. First, the indenting in getVehicles<\/span>\u00a0. It looks bad. It is bad. And if we\u00a0want to add\u00a0more callbacks, it gets worse. Second, we are using error handling code\u00a0in each individual callback.<\/p>\n
We can rewrite this code to use Q<\/span>\u00a0.<\/p>\n
'use strict';\r\n\r\nvar Q        = require('q');\r\nvar mongoose = require('mongoose');\r\nvar Schema   = mongoose.Schema;\r\n\r\nvar UserSchema = new Schema({\r\n  email    : { type: String },\r\n  firstName: { type: String },\r\n});\r\nvar User = mongoose.model('User', UserSchema);\r\n\r\nvar CarSchema = new Schema({\r\n  ownerId       : { type: Schema.Types.ObjectId, ref: 'User' },\r\n  colour        : { type: String },\r\n  numberOfWheels: { type: Number },\r\n});\r\nvar Car = mongoose.model('Car', CarSchema);\r\n\r\nvar BicycleSchema = new Schema({\r\n  ownerId       : { type: Schema.Types.ObjectId, ref: 'User' },\r\n  colour        : { type: String },\r\n  numberOfWheels: { type: Number },\r\n});\r\nvar Bicycle = mongoose.model('Bicycle', BicycleSchema);\r\n\r\nmongoose.connect('mongodb:\/\/localhost\/blogtest');\r\n\r\nvar email = 'rupert@example.com';\r\n\r\nfunction getVehicles(email) {\r\n  var foundCars, foundUser;\r\n  return Q(User.findOne({ email: email }).exec())\r\n  .then(function(user) {\r\n    foundUser = user;\r\n    return Q(Car.find({ ownerId: user._id }).exec())\r\n  })\r\n  .then(function(cars) {\r\n    foundCars = cars;\r\n    return Q(Bicycle.find({ ownerId: foundUser._id }).exec())\r\n  })\r\n  .then(function(bicycles) {\r\n    return {\r\n      bicycles: bicycles,\r\n      cars: foundCars\r\n    };\r\n  });\r\n}\r\n\r\ngetVehicles(email)\r\n.then(function(vehicles) {\r\n  console.log(vehicles);\r\n})\r\n.catch(function(err) {\r\n  console.error('Something went wrong: ' + err);\r\n})\r\n.done(function() {\r\n  mongoose.disconnect();\r\n});\r\n<\/pre>\n
The getVehicles<\/span>\u00a0 function now returns the\u00a0promise created on line 33. The user is found, then the cars, then the bicycles, and then the promise resolves to the same object as in the first listing.<\/p>\n
This version of\u00a0getVehicles<\/span>\u00a0looks better regarding both indenting and error handling. The indenting stays on the same level, and the error handling – no matter where the error\u00a0happens – is done by the catch<\/span>\u00a0 block after calling getVehicles<\/span>\u00a0.<\/p>\n
However, there is also a disadvantage to this code. The vars\u00a0from a then<\/span>\u00a0 block\u00a0can’t be used in the next then<\/span>\u00a0 block, because each consecutive then<\/span>\u00a0 block has its own function scope. That’s why we have to assign user<\/span>\u00a0 and cars<\/span>\u00a0 to helper vars outside their scope.<\/p>\n
We can do better. Let’s look at our\u00a0improved version of getVehicles<\/span>\u00a0.<\/p>\n
function getVehicles(email) {\r\n  return Q(User.findOne({ email: email }).exec())\r\n  .then(function(user) {\r\n    return Q.all([\r\n      Q(Bicycle.find({ ownerId: user._id }).exec()),\r\n      Q(Car.find({ ownerId: user._id }).exec())\r\n    ]);\r\n  })\r\n  .then(function(results) {\r\n    return {\r\n      bicycles: results[0],\r\n      cars: results[1]\r\n    };\r\n  });\r\n}\r\n\r\n<\/pre>\n
We use Q.all(arrayOfPromises)<\/span>\u00a0to retrieve\u00a0both the cars<\/span>\u00a0 and the bicycles<\/span>\u00a0 in one go. Q.all<\/span>\u00a0 resolves to a list of fulfilment values, which end up in an array in the then<\/span>\u00a0 block.<\/p>\n
This both makes the code shorter and gets rid of the helper variables. There is one minor improvement that we can still make.<\/p>\n
function getVehicles(email) {\r\n  return Q(User.findOne({ email: email }).exec())\r\n  .then(function(user) {\r\n    return [\r\n      Q(Bicycle.find({ ownerId: user._id }).exec()),\r\n      Q(Car.find({ ownerId: user._id }).exec())\r\n    ];\r\n  })\r\n  .spread(function(bicycles, cars) {\r\n    return {\r\n      bicycles: bicycles,\r\n      cars: cars\r\n    };\r\n  });\r\n}\r\n<\/pre>\n
We replace .then<\/span>\u00a0 by .spread<\/span>\u00a0, so the bicycles<\/span>\u00a0 and cars<\/span>\u00a0 end up in one argument each, instead of an array of results. Additionally, because .spread<\/span>\u00a0 calls .all<\/span>\u00a0 initially, we can now remove Q.all and return just the\u00a0array of promises. This code is more readable and maintainable than the previous version.<\/p>\n","protected":false},"excerpt":{"rendered":"
This post will teach you how to write promise-based mongoose code, using Kris Kowal’s Q library. You can also find the code below in the `src\/mongoose-and-q` dir of my blog code repository. We\u00a0have a mongo database with 3 collections: users, … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[176,177,10],"tags":[189,229,188,230,227,228,226,178,179,183],"class_list":["post-396","post","type-post","status-publish","format-standard","hentry","category-javascript","category-node-js","category-software-development","tag-array-of-promises","tag-callback","tag-javascript-2","tag-mongo","tag-mongoose","tag-node","tag-node-js","tag-promise","tag-promises","tag-q"],"_links":{"self":[{"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/posts\/396"}],"collection":[{"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/comments?post=396"}],"version-history":[{"count":21,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/posts\/396\/revisions"}],"predecessor-version":[{"id":439,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/posts\/396\/revisions\/439"}],"wp:attachment":[{"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/media?parent=396"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/categories?post=396"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/joost.vunderink.net\/blog\/wp-json\/wp\/v2\/tags?post=396"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}