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Data Associations with Mongoose

Why is this important?

This workshop is important because:

  • Real-world data usually consists of different types of things that are related to each other in some way. An invoicing app might need to track employees, customers, and accounts. A food ordering app needs to know about restaurants, menus, and its users!

  • We've seen that when data is very simple, we can combine it all into one model. When data is more complex or more loosely related, we often create two or more related models.

  • Understanding how to plan for, set up, and use related data will help us build more full-featured applications.

What are the objectives?

After this workshop, developers will be able to:

  • Diagram one-to-one, one-to-many, and many-to-many data associations.
  • Compare and contrast embedded & referenced data.
  • Design nested server routes for associated resources.
  • Build effective Mongoose queries for associated resources.

Where should we be now?

Before this workshop, developers should already be able to:

  • Use Mongoose to code Schemas and Models for single resources.
  • Create, Read, Update, and Delete data with Mongoose.

Numerical Categories for Relationships


Each person has one brain, and each (living human) brain belongs to one person.

one to one erd example

One-to-one relationships can sometimes just be modeled with simple attributes. A person and a brain are both complex enough that we might want to have their data in different models, with lots of different attributes on each.


Each leaf "belongs to" the one tree it grew from, and each tree "has many" leaves.

one to many erd example


Each student "has many" classes they attend, and each class "has many" students.

many to many erd example

Entity Relationship Diagrams

Entity relationship diagrams (ERDs) represent information about the numerical relationships between data, or entities.

entity relationship diagram example

Note: In the example above, all of the Item1, Item2, Item3 under each heading are standing in for attributes.

More guidelines for ERDs

Check for Understanding

Come up with an example of related data. Draw the ERD for your relationship, including a few attributes for each model.

Association Categories for Mongoose

Embedded Data is directly nested inside of other data. Each record has a copy of the data.

It is often efficient to embed data because you don't have to make a separate request or a separate database query -- the first request or query gets you all the information you need.

Referenced Data is stored as an id inside other data. The id can be used to look up the information. All records that reference the same data look up the same copy.

It is usually easier to keep referenced records consistent because the data is only stored in one place and only needs to be updated in one place.

image Source: MongoDB docs

While the question of one-to-one, one-to-many, or many-to-many is often determined by real-world characteristics of a relationship, the decision to embed or reference data is a design decision.

There are tradeoffs, such as between efficiency and consistency, depending on which one you choose.

When using Mongo and Mongoose, though, many-to-many relationships often involve referenced associations, while one-to-many often involve embedding data.

Check for Understanding

How would you design the following? Draw an ERD for each set of related data? Can you draw an ERD for each?

  • Users with many Tweets?
  • Foods with many Ingredients?

Implementation: Referenced

1) Set Up Structure with Schemas

var ingredientSchema = new Schema({
  title: {
    type: String,
    default: ""
  origin: {
    type: String,
    default: ""

var foodSchema = new Schema({
  name: {
    type: String,
    default: ""
  ingredients: [{
    type: Schema.Types.ObjectId,  //REFERENCING :D
    ref: 'Ingredient'

Check out the value associated with the ingredients key inside the food schema. Here's how it's set up as an array of referenced ingredients:

  • [] lets the food schema know that each food's ingredients attribute will hold an array.
  • The object inside the [] describes what kind of elements the array will hold.
  • Giving type: Schema.Types.ObjectId tells the schema the ingredients array will hold ObjectIds. That's the type of that unique _id that Mongo automatically generates for us (something like 55e4ce4ae83df339ba2478c6).
  • ref: Ingredient tells the schema we will only be putting ObjectIds of Ingredient documents inside the ingredients array.

2) Manipulate Data with Models

Once schemas are defined, we can compile them all into active models so we can start creating documents!

/* Compiling models from the above schemas */
var Food = mongoose.model('Food', foodSchema);
var Ingredient = mongoose.model('Ingredient', ingredientSchema);

Here's how we'd take our models for a spin and make two objects to test out creating an Ingredient document and a Food document.

/* make a new Ingredient document */
var cheddar = new db.Ingredient ({
 title: 'cheddar cheese',
 origin: 'Wisconsin'
/* make a new Food document */
var cheesyQuiche = new db.Food ({
  name: 'Quiche',
  ingredients: []

Don't forget to save your work!, savedCheese) {
  if (err) {
    return console.log(err);
  } else {
    console.log('cheddar saved successfully');

cheesyQuiche.ingredients.push(cheddar);   // associated!, savedCheesyQuiche) {
  if (err) {
    return console.log(err);
  } else {
    console.log('cheesyQuiche food is ', savedCheesyQuiche);

Note that we push the cheddar ingredient document into the cheesyQuiche ingredients array. We already told the Food Schema that we will only be storing ObjectIds, though, so cheddar gets converted to its unique _id when it's pushed in!

Check In

This is the log text after executing the code we've written thus far:

cheesyQuiche food is { __v: 0,
  name: 'Quiche',
  _id: 55e4eb857d6157f4d41a2981,
  ingredients: [ 55e4eb857d6157f4d41a2980 ] }

cheesy quiche saved successfully

What are we looking at?

click for line-by-line explanation
  1. Line 1: __v represents the number of times the document has been accessed.

  2. Line 2: The name property of the Food document we have created.

  3. Line 4: The unique _id created by Mongo for our Food document.

  4. Line 5: The ingredients array, with a single ObjectId that is associated with our Ingredient document.

Mongoose is happy to show just the ObjectId associated with each ingredient in the food's ingredients array. When we need the Ingredient document data, we have to ask for it explicitly.

3) Pull Data in With .populate()

When we want to get full information from an Ingredient document we have inside the Food document ingredients array, we use a method called .populate().

db.Food.findOne({ name: 'Quiche' })
  .populate('ingredients')    // <- pull in ingredient data
  .exec(function(err, food) {
    if (err){
    if (food.ingredients.length > 0) {
      console.log('/nI love ' + + ' for the '+ food.ingredients[0].title);
    else {
      console.log( + ' has no ingredients.');
    console.log('what was that food?', food);
Click to go over this method call line by line:
  1. Line 1: We call a method to find only one Food document that matches the name: Quiche.

  2. Line 2: We ask the ingredients array within that Food document to fetch the actual Ingredient document instead of just its ObjectId.

  3. Line 3: When we use find without a callback, then populate, like here, we can put a callback inside an .exec() method call. Technically we have made a query with find, but only executed it when we call .exec().

  4. Lines 4-15: If we have any errors, we will log them. Otherwise, we can display the entire Food document including the populated ingredients array.

  5. Line 9 demonstrates that we are able to access both data from the original Food document we found and the referenced Ingredient document we summoned.

Click to see the output from the above findOne() method call with populate.
  _id: 55e4eb857d6157f4d41a2981,
  name: 'Quiche',
  __v: 1,
  ingredients: [
       _id: 55e4eb857d6157f4d41a2980,
       title: 'cheddar cheese',
       origin: 'Wisconson',
       __v: 0

I love Quiche for the cheddar cheese

Now, instead of seeing only the ObjectId that pointed us to the Ingredient document, we can see the entire Ingredient document.

Independent Practice: Foods & Ingredients

Get it:

  • fork and clone this repo
  • start up mongoDB with mongod
  • cd into the folder starter-code in this directory
  • npm install to install all the dependencies from package.json
  • node console.js to enter into a REPL where you can interact with your DB. All the models will be nested inside an object called db.


  • save your successful code into a file for each step
  • <command> + <up> will bring you to the last thing you entered in the repl


  1. Create 3 ingredients.
  2. Create a food that references those ingredients.
  3. List all the foods.
  4. List all the ingredient data for a food.

Routes for Referenced Data

When you need full information about a food, remember to pull ingredient data in with populate. Here's an example:

index of all foods

// send all information for all foods
app.get('/api/foods/', function (req, res) {
  Food.find({ })
    .exec(function(err, foods) {
      if (err) {
      console.log('found and populated all foods: ', foods);

Many APIs don't populate all referenced information before sending a response. For instance, the Spotify API is riddled with ids that developers can use to make a second request if they want more of the information.

Check for Understanding

On which of the following routes are you most likely to populate all the ingredients of a food you look up?

HTTP Verb Path Description
GET /foods Get all foods
POST /foods Create a food
GET /foods/:id Get a food
DELETE /foods/:id Delete a food
GET /foods/:food_id/ingredients Get all ingredients from a food

Implementation: Embedded

Imagine you have a database of Users, each with many embedded Tweets. If you needed to update or delete a tweet, you would first need to find the correct user, then the tweet to update or delete.

1) Set Up Structure with Schemas

var tweetSchema = new Schema({
  text: String,
  date: Date
var userSchema = new Schema({
  name: String,
  // embed tweets in user
  tweets: [tweetSchema]

The tweets: [tweetSchema] line sets up the embedded data association. The [] tells the schema to expect a collection, and tweetSchema (or Tweet.schema if you had a Tweet model defined already) tells the schema that the collection will hold embedded documents of type Tweet.

2) Manipulate Data with Models

var User = mongoose.model("User", userSchema);
var Tweet = mongoose.model("Tweet", tweetSchema);

Independent Practice: Users & Tweets

  1. Create a user.

  2. Create tweets embedded in that user.

  3. List all the users.

  4. List all tweets of a specific user.

Routes for Embedded Data

create tweet

// create tweet embedded in user'/api/users/:userId/tweets', function (req, res) {
  // set the value of the user id
  var userId = req.params.userId;

  // store new tweet in memory with data from request body
  var newTweet = new Tweet(req.body.tweet);

  // find user in db by id and add new tweet
  User.findOne({_id: userId}, function (err, foundUser) {
    foundUser.tweets.push(newTweet); (err, savedUser) {

update tweet

// update tweet embedded in user
app.put('/api/users/:userId/tweets/:id', function (req, res) {
  // set the value of the user and tweet ids
  var userId = req.params.userId;
  var tweetId =;

  // find user in db by id
  User.findOne({_id: userId}, function (err, foundUser) {
    // find tweet embedded in user
    var foundTweet =;
    // update tweet text and completed with data from request body
    foundTweet.text = req.body.tweetText; = new Date(req.body.tweetDate); (err, savedUser) {

Route Design

Remember RESTful routing? It's the most popular modern convention for designing resource paths for nested data. Here is an example of an application that has routes for Store and Item models:

RESTful Routing

HTTP Verb Path Description Key Mongoose Method(s)
GET /stores Get all stores
click for ideas.find
POST /stores Create a store
click for ideasnew, .save
GET /stores/:id Get a store
click for ideas.findOne
DELETE /stores/:id Delete a store
click for ideas.findOne, .remove, .findOneAndRemove
GET /stores/:store_id/items Get all items from a store
click for ideas.findOne, (.populate if referenced)
POST /stores/:store_id/items Create an item for a store
click for ideas.findOne, new, .save
GET /stores/:store_id/items/:item_id Get an item from a store
click for ideas.findOne
DELETE /stores/:store_id/items/:item_id Delete an item from a store
click for ideas.findOne, .remove

In routes, avoid nesting resources more than one level deep.

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