Mongoose Relationships Adding References

Learning objective: By the end of this lesson, students will be able to create relationships through referenced documents in MongoDB.

Forming relationships through referencing

Our todoSchema includes an assignee field, which holds a reference to a User:

// models/todo.js
const todoSchema = new mongoose.Schema({
  text: String,
  isComplete: Boolean,
  assignee: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});

This field allows us to link documents in separate collections. The schema is complete, but forming a relationship between Todo and User documents will still require an additional operation. We still need to update the Todo document to include the ObjectId of a User in the assignee field. It’s this step that establishes the direct connection between two documents.

Creating a new user

To demonstrate this, we’ll need to add a new user to our database.

Import the User model in queries.js:

const User  = require('./models/user.js')

Add the following function to queries.js at the bottom of the Query Functions section:

const createUser = async () => {
  const userData = {
    name: "Alex",
    email: "alex@mail.com",
  };
  const user = await User.create(userData);
  console.log("New user:", user);
};

Next, call createUser with the runQueries function:

const runQueries = async () => {
  console.log('Queries running.');
  await createUser();
};

Run the queries.js file with the following command:

node queries.js

You should see a similar output in your terminal:

New user: {
  name: 'Alex',
  email: 'alex@mail.com',
  _id: new ObjectId('6581d503b8b46a5314295d66'),
  __v: 0
}

Add a reference

Now that we have a user, let’s build a function to form the relationship between user and a todo.

To do this we’ll use the findByIdAndUpdate() mongoose method, which takes three arguments:

• the first argument is the id of the document to look up.
• the second is an object with desired updates.
• the third is for options (in this case, new: true specifies that the function should return the modified document rather than the original).

Create the following function in queries.js at the bottom of the Query Functions section:

const assignTodo = async () => {
  const todoId = '657b25adc8146427465857d7';
  const userId = '6581d503b8b46a5314295d66';

  const updatedTodo = await Todo.findByIdAndUpdate(
    todoId,
    { assignee: userId },
    { new: true }
  );

  console.log('Updated document:', updatedTodo);
};

🚨 Make sure that you replace todoId and userId with actual ObjectIds from your database.

Next, call upon assignTodo within the runQueries function:

const runQueries = async () => {
  console.log('Queries running.');
  await assignTodo();
};

Run the queries.js file with the following command:

node queries.js

You should see a similar output in your terminal:

Updated document: {
  _id: new ObjectId('657b25adc8146427465857d7'),
  text: 'Learn React',
  isComplete: false,
  __v: 6,
  subtasks: [],
  assignee: new ObjectId('6581d503b8b46a5314295d66')
}

In the example above, the ObjectId of a user document has been added to the assignee property of a todo document. When the todo document needs information about the assigned user, the appropriate document can be retrieved with this ObjectId.