1. Definition

The $project stage is used to reshape documents by selecting, including, excluding, or modifying fields in the output.

2. Simple Understanding

👉 It works like selecting columns in SQL
👉 You control what fields appear in the final result

3. When to Use

• When you want specific fields only
• When removing unnecessary data
• When renaming fields
• When transforming data structure

4. Where it is Used

• Aggregation pipelines
• API responses
• Data transformation
• Frontend data shaping

5. Why We Use It

• Reduce data size
• Improve performance
• Customize output
• Hide sensitive fields

6. How It Works

• 1 → include field
• 0 → exclude field
• Can rename fields
• Can create computed fields

7. Basic Syntax

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

8. Real Example

// Show only name and role
db.users.aggregate([
  {
    $project: {
      name: 1,
      role: 1,
      _id: 0
    }
  }
]);

// Rename field
db.users.aggregate([
  {
    $project: {
      username: "$name",
      _id: 0
    }
  }
]);

// Create computed field
db.products.aggregate([
  {
    $project: {
      name: 1,
      discountedPrice: { $multiply: ["$price", 0.9] }
    }
  }
]);

9. Explanation

• $project controls output fields
• 1 includes, 0 excludes fields
• Can rename and transform data
• Helps optimize response size

10. Advantages

• Custom output structure
• Reduces data transfer
• Improves performance
• Flexible transformations

11. Disadvantages

• Complex transformations can be hard to read
• Requires proper field handling