Mongoose Query prototype.selected() API

Last Updated : 17 Nov, 2022

Mongoose is an Object Data Modeling (ODM) library for MongoDB. It defines a strongly-typed schema, with default values and schema validations which are later mapped to a MongoDB document.

Mongoose Query API selected method is used to determine whether any selection is made in the Mongoose Query. It gives a Boolean response, i.e, “true” if any selection is made, else “false”. Let’s understand more about this with some examples.

Syntax:

Query.prototype.selected()

Parameters: It does not accept any parameter.

Return type: It returns a Boolean response.

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 completing 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 selected() method to check if a selection is made in the Mongoose Query. It will return “true” as we select the “name” property in the documents fetched from the DB.

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, 
        select: false
    }, 
    age: { 
        type: Number, 
    } 
}); 
  
const personsArray = [ 
    { 
        name: 'Luffy', 
        age: 22 
    }, 
    { 
        name: 'Nami', 
        age: 30 
    }, 
    { 
        name: 'Zoro', 
        age: 15 
    } 
] 
  
const Person = mongoose.model('Person', personSchema); 
  
(async () => { 
    const query = Person.find().select('name') 
    console.log(query.selected()); 
})()

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 selected() method to check if a selection is made in the Mongoose Query. It will return “false” as we first select the “name” property and then deselect it using the Mongoose Query chaining technique.

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, 
        select: false
    }, 
    age: { 
        type: Number, 
    } 
}); 
  
const personsArray = [ 
    { 
        name: 'Luffy', 
        age: 22 
    }, 
    { 
        name: 'Nami', 
        age: 30 
    }, 
    { 
        name: 'Zoro', 
        age: 15 
    } 
] 
  
const Person = mongoose.model('Person', personSchema); 
  
(async () => { 
    const query = Person.find().select('name').select('-name') 
    console.log(query.selected()); 
})() 