Mongoose Query prototype.transform() API

Mongoose is a MongoDB object modeling and handling for a node.js environment. Mongoose Query transform() method is used to transform the document returned from the mongoose query response. The transformation could be useful when we need to add a new property to the documents loaded. Let’s understand more about this with some examples.

Syntax:

Query.prototype.transform()

Parameters: It accepts a single parameter: that is described below:

• fn: It is a function that runs that transforms the query result

Return type: It returns the Query object.

Creating node application And Installing Mongoose:

Step 1: Create a node application using the following command:

mkdir folder_name
cd folder_name
npm init -y
touch main.js

Step 2: After creating the ReactJS application, Install the required module using the following command:

npm install mongoose

Project Structure: It will look like the following.

GUI Representation of the  Database using MongoDB Compass: Currently, the collection has no data.

Example 1: In this example, we will use the Query API transform() method to set the current date to the document returned.

Filename: main.js

Javascript

const mongoose = require('mongoose')

// Database connection
mongoose.connect('mongodb://localhost:27017/query-helpers', {
    dbName: 'event_db',
    useNewUrlParser: true,
    useUnifiedTopology: true
}, err => err ? console.log(err) :
    console.log('Connected to database'));

const personSchema = new mongoose.Schema({
    name: {
        type: String,
    },
    age: {
        type: Number,
    }
});

const personsArray = [
    {
        name: 'John',
        age: 22
    }
]

const Person = mongoose.model('Person', personSchema);

(async () => {
    await Person.insertMany(personsArray);

    const persons = await Person.find().transform(per => {
        return {
            ...per,
            currentDate: new Date()
        }
    })

    console.log(persons);
})()

Step to Run Application: Run the application using the following command from the root directory of the project:

node main.js

Output: We see that the value remains unchanged in the result.

GUI Representation of the Database using MongoDB Compass:

Example 2: In this example, we will use the Query API transform() method to set the count of the documents returned.

Filename: main.js

Javascript

const mongoose = require('mongoose')

// Database connection
mongoose.connect('mongodb://localhost:27017/query-helpers', {
    dbName: 'event_db',
    useNewUrlParser: true,
    useUnifiedTopology: true
}, err => err ? console.log(err) :
    console.log('Connected to database'));

const personSchema = new mongoose.Schema({
    name: {
        type: String,
    },
    age: {
        type: Number,
    }
});

const personsArray = [
    {
        name: 'John',
        age: 22
    }
]

const Person = mongoose.model('Person', personSchema);

(async () => {
    await Person.insertMany(personsArray);
    const persons = await Person.find().transform(per => {
        return {
            ...per,
            count: per.length
        }
    })

    console.log(persons);
})()

Step to Run Application: Run the application using the following command from the root directory of the project:

node main.js

Output: We see that the value remains unchanged in the result.

GUI Representation of the  Database using MongoDB Compass:

Reference: https://mongoosejs.com/docs/api/query.html#query_Query-transform