$project stage for shaping output in MongoDB

Introduction

The $project stage lets you choose which fields to keep, remove, or create in your MongoDB query results. It helps you shape the output to show only what you want.

You want to show only a few fields from each document, hiding the rest.

You want to rename fields or create new fields based on existing data.

You want to calculate new values for each document before showing results.

You want to simplify the output to make it easier to read or use in your app.

Syntax

MongoDB

db.collection.aggregate([
  { $project: {
      field1: 1,
      field2: 1,
      newField: { $someOperator: "$existingField" },
      _id: 0
  }}
])

Use 1 to include a field and 0 to exclude it.

You can create new fields using expressions inside $project.

Examples

This shows only the name and age fields, hiding the _id field.

MongoDB

db.users.aggregate([
  { $project: { name: 1, age: 1, _id: 0 } }
])

This creates a new field totalPrice by multiplying price and quantity.

MongoDB

db.orders.aggregate([
  { $project: { totalPrice: { $multiply: ["$price", "$quantity"] }, _id: 0 } }
])

This renames the name field to productName and includes price.

MongoDB

db.products.aggregate([
  { $project: { productName: "$name", price: 1, _id: 0 } }
])

Sample Program

This query creates a new field fullName by joining firstName and lastName, includes department, and hides _id.

MongoDB

db.employees.aggregate([
  { $project: {
      fullName: { $concat: ["$firstName", " ", "$lastName"] },
      department: 1,
      _id: 0
  }}
])

Important Notes

If you include any field with 1, MongoDB excludes all others by default except _id.

To hide _id, you must explicitly set _id: 0.

You can use many MongoDB expressions inside $project to transform data.

Summary

$project shapes the output by selecting, renaming, or creating fields.

Use 1 to keep fields, 0 to hide them.

You can build new fields using expressions like $concat or $multiply.