Mongoose Query.prototype.pre() API

The Mongoose Query API pre() method is used to add a pre-middleware to the mongoose query instance, which can be used in performing pre-query operations.

Syntax:

Query.prototype.pre(fn)

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

• fn: It is a middleware function that is used to perform pre-query operations

Return type: It returns a Query 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 query filters to the console.

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,
    }
});

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

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

(async () => {
    let query = Person.find({ age: 20 })
    query.pre(function middleware() {
        console.log(this.getFilter());
    })
    await query.exec()
})()

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 use this method to log the whole query to the console.

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,
    }
});

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

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

(async () => {
    let query = Person.find({ name: 'Luffy' })
        .where('age').equals(20)
    query.pre(function middleware() {
        console.log(this.getQuery());
    })
    await query.exec()
})()

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/api/query.html#query_Query-pre