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
const mongoose = require( 'mongoose' )
{
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
const mongoose = require( 'mongoose' )
{
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
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