Mongoose is a MongoDB object modeling and handling for a node.js environment. Mongoose Query transform() method is used to transform the document returned from the mongoose query response. The transformation could be useful when we need to add a new property to the documents loaded. Let’s understand more about this with some examples.
Syntax:
Query.prototype.transform()
Parameters: It accepts a single parameter: that is described below:
- fn: It is a function that runs that transforms the query result
Return type: It returns the Query object.
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 creating the ReactJS application, Install the required module using the following command:
npm install mongoose
Project Structure: It will look like the following.
GUI Representation of the Database using MongoDB Compass: Currently, the collection has no data.
Example 1: In this example, we will use the Query API transform() method to set the current date to the document returned.
Filename: main.js
const mongoose = require( 'mongoose' )
// Database connection 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: 'John' ,
age: 22
}
] const Person = mongoose.model( 'Person' , personSchema);
(async () => { await Person.insertMany(personsArray);
const persons = await Person.find().transform(per => {
return {
...per,
currentDate: new Date()
}
})
console.log(persons);
})() |
Step to Run Application: Run the application using the following command from the root directory of the project:
node main.js
Output: We see that the value remains unchanged in the result.
GUI Representation of the Database using MongoDB Compass:
Example 2: In this example, we will use the Query API transform() method to set the count of the documents returned.
Filename: main.js
const mongoose = require( 'mongoose' )
// Database connection 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: 'John' ,
age: 22
}
] const Person = mongoose.model( 'Person' , personSchema);
(async () => { await Person.insertMany(personsArray);
const persons = await Person.find().transform(per => {
return {
...per,
count: per.length
}
})
console.log(persons);
})() |
Step to Run Application: Run the application using the following command from the root directory of the project:
node main.js
Output: We see that the value remains unchanged in the result.
GUI Representation of the Database using MongoDB Compass:
Reference: https://mongoosejs.com/docs/api/query.html#query_Query-transform