Mongoose Query.prototype.toConstructor() API

The Mongoose Query API.prototype.toConstructor() method of the Mongoose API is used on the Query objects. It allows us to customize the query constructor. Using this method we can make reusable constructors with all the configured properties and options. Let us understand toConstructor() method using an example.

Syntax:

query.toConstrutor();

Parameters: This method does not accept any parameter.

Return Value: This method returns the sub-class of Query. On its reference, we can access the callback function.

Setting up Node.js Mongoose Module:

Step 1: Create a Node.js application using the following command:

npm init

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

npm install mongoose

Project Structure: The project structure will look like this: 

Database Structure: The database structure will look like this, the following documents are present in the collection.

Example 1: The below example illustrates the basic functionality of the Mongoose Query toConstructor() method, using asynchronous function.

Filename: app.js

Javascript

// Require mongoose module
const mongoose = require("mongoose");

// Set Up the Database connection
const URI = "mongodb://localhost:27017/geeksforgeeks"

const connectionObject = mongoose.createConnection(URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
});

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

const Student = connectionObject.model(
    'Student', studentSchema
);

(async () => {
    const query = Student.find({ name: 'Student1' });
    const Student1 = query.toConstructor();
    Student1().exec((error, result) => {
        if (error) {
            console.log('Error -', error);
        } else {
            console.log('Result -', result);
        }
    })
})();

Step to run the program: To run the application execute the below command from the root directory of the project:

node app.js

Output:

Result - [
  {
    _id: new ObjectId("63a40a1065e8951038a391b1"),
    name: 'Student1',
    age: 30,
    rollNumber: 9,
    __v: 0
  }
]

Example 2: The below example illustrates the basic functionality of the Mongoose Query toConstructor() method, using callback function.

Filename: app.js

Javascript

// Require mongoose module
const mongoose = require("mongoose");

// Set Up the Database connection
const URI = "mongodb://localhost:27017/geeksforgeeks"

const connectionObject = mongoose.createConnection(URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
});

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

const Student = connectionObject.model(
    'Student', studentSchema
);

const query = Student.findByIdAndUpdate(
    { id: '63a4a9a207370cdcd1961b2c' },
    { name: 'Student2' }
);

const Student2 = query.toConstructor();
Student2().then(result => {
    console.log('Result -', result);
}).catch(error => { console.log(error) });

Step to run the program: To run the application execute the below command from the root directory of the project:

node app.js

Output:

GUI Representation of the Database using Robo3T GUI tool:

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