Mongoose Schemas Instance methods

Mongoose is a MongoDB object modeling and handling for a node.js environment. Mongoose Schemas Instance methods refer to the methods used by Mongoose documents. These methods can be both, built-in and custom. 

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

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.

Example 1: In this example, we will create a custom mongoose instance method to show all the documents with the type “dog”.

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 animalSchema = new mongoose.Schema(
    { name: String, type: String }, {

    // Assign a function to the "methods" object of our
    // animalSchema through schema options.
    // By following this approach, there is no need to
    // create a separate TS type to define the type
    // of the instance functions.
    methods: {
        findSimilarTypes(cb) {
            return mongoose.model('Animal').find(
                { type: this.type }, cb);
        }
    }
}
);

const Animal = mongoose.model("Animal", animalSchema);

const animals = [
    {
        name: 'bond',
        type: 'dog'
    },
    {
        name: 'cavine',
        type: 'cat'
    }
]

const dog = new Animal({ type: 'dog' });

Animal.insertMany(animals, (err, res) => {
    dog.findSimilarTypes((err, dogs) => {
        console.log(dogs);
    });
})

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

node main.js

Output: 

Example 2:  In this example, we will create a custom mongoose instance method to show all the documents with the type “cat”.

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 animalSchema = new mongoose.Schema({ name: String, type: String }, {

        // Assign a function to the "methods" object
        // of our animalSchema through schema options.
        // By following this approach, there is no need
        // to create a separate TS type to define the type
        // of the instance functions.
        methods: {
            findSimilarTypes(cb) {
                return mongoose.model('Animal').find({ type: this.type }, cb);
            }
        }
    }
);

const Animal = mongoose.model("Animal", animalSchema);

const animals = [
    {
        name: 'bond',
        type: 'dog'
    },
    {
        name: 'cavine',
        type: 'cat'
    }
]

const cat = new Animal({ type: 'cat' });

Animal.insertMany(animals, (err, res) => {
    cat.findSimilarTypes((err, cats) => {
        console.log(cats);
    });
})

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

node main.js

Output:

Reference: https://mongoosejs.com/docs/guide.html#methods