Mongoose Schema.prototype.pre() API

Last Updated : 24 Jan, 2023

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() 
})()