Mongoose Query.prototype.pre() API
Last Updated :
24 Jan, 2023
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
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,
}
});
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
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,
}
});
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
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