Mongoose Schema.prototype.pre() API

The Mongoose Schema API pre() method is used to add a pre-hook to the mongoose Schema methods and can be used to perform pre Schema method operations. 

Syntax:

Schema.prototype.pre(methodName, options, callback)

Parameters: It accepts the following parameters as mentioned above and described below:

• methodName: It denotes the name of the Schema method name, or regex for the method name, to apply the pre middleware to 
• options: It is an optional mongoose object that contains options.document and options.query.
• callback: It is a callback function that accepts the parameter next.

Return type: It returns a Schema object as a 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 Node.js application, Install the required module using the following command:

npm install mongoose

Example 1: In this example, we will use this method to log the filters applied to the mongoose query.

Filename: main.js

Javascript

// Importing the module
const mongoose = require('mongoose')

// Creating the 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,
    }
});

personSchema.pre(/^find/, function (next) {
    console.log(this.getFilter());
});

const personsArray = [
    {
        name: 'Luffy',
        age: 20
    },
    {
        name: 'Nami',
        age: 20,
    },
    {
        name: 'Zoro',
        age: 35
    }
]

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

(async () => {
    await Person.insertMany(personsArray)
    await Person.find({ name: "Luffy", age: 20 })
})()

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

node main.js

Output:

GUI Representation of the  Database using MongoDB Compass:

Example 2: In this example, we will use this method to update the name of a mongoose document before saving it to the MongoDB

Filename: main.js

Javascript

// Importing the module
const mongoose = require('mongoose')

// Creating the 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,
    }
});

personSchema.pre('save', function (next) {
    if (this.name === 'Luffy') {
        this.name = 'Nami'
    }

    next()
});

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

(async () => {
    const person = new Person({ name: 'Luffy', age: '19' })
    await person.save()
})()

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

node main.js

GUI Representation of the  Database using MongoDB Compass:

Reference: https://mongoosejs.com/docs/api/schema.html#schema_Schema-pre