$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.

Practice

1. What does the $project stage do in a MongoDB aggregation pipeline?

easy

A. It filters documents based on conditions.

B. It shapes the output by selecting, renaming, or creating fields.

C. It sorts the documents in ascending or descending order.

D. It groups documents by a specified key.

Solution

1. Step 1: Understand the purpose of $project

   The $project stage is used to control which fields appear in the output documents and how they are shaped.

2. Step 2: Compare with other stages

   Filtering is done by $match, sorting by $sort, and grouping by $group. So $project is for shaping output fields.

3. Final Answer:

   It shapes the output by selecting, renaming, or creating fields. -> Option B

4. Quick Check:

   $project shapes output = A [OK]

Hint: Remember: $project controls output fields only [OK]

Common Mistakes:

• Confusing $project with $match for filtering
• Thinking $project sorts documents
• Mixing $project with $group for aggregation

2. Which of the following is the correct syntax to include only the name and age fields in the output using $project?

easy

A. { $project: { name: 1, age: 1 } }

B. { $project: { name: 0, age: 0 } }

C. { $project: { name: true, age: true } }

D. { $project: { name: 'include', age: 'include' } }

Solution

1. Step 1: Recall $project syntax for inclusion

   To include fields, set them to 1. To exclude, set to 0. Boolean or strings are invalid.

2. Step 2: Check each option

   { $project: { name: 1, age: 1 } } uses 1 for inclusion correctly. { $project: { name: 0, age: 0 } } excludes fields. { $project: { name: true, age: true } } uses true which is invalid. { $project: { name: 'include', age: 'include' } } uses strings which is invalid.

3. Final Answer:

   { $project: { name: 1, age: 1 } } -> Option A

4. Quick Check:

   Include fields = 1 in $project [OK]

Hint: Use 1 to include fields, 0 to exclude in $project [OK]

Common Mistakes:

• Using true/false instead of 1/0
• Mixing inclusion and exclusion in same $project
• Using strings instead of numbers for field selection

3. Given the collection documents:

{ "name": "Alice", "age": 25, "city": "NY" }

What will be the output of this aggregation?

[{ $project: { fullName: "$name", ageInMonths: { $multiply: ["$age", 12] }, _id: 0 } }]

medium

A. [{ "fullName": "Alice", "ageInMonths": 25 }]

B. [{ "name": "Alice", "ageInMonths": 300 }]

C. [{ "fullName": "Alice", "age": 25 }]

D. [{ "fullName": "Alice", "ageInMonths": 300 }]

Solution

1. Step 1: Understand field renaming and calculation

   The fullName field is created by copying name. The ageInMonths is calculated by multiplying age by 12 (25*12=300). The _id is excluded.

2. Step 2: Check output fields

   Output has fullName with "Alice" and ageInMonths with 300. No name or age fields remain.

3. Final Answer:

   [{ "fullName": "Alice", "ageInMonths": 300 }] -> Option D

4. Quick Check:

   Rename + multiply age = output [OK]

Hint: Use $multiply to calculate new fields in $project [OK]

Common Mistakes:

• Expecting original fields to remain without including them
• Confusing field names after renaming
• Forgetting to exclude _id if not wanted

4. Identify the error in this $project stage:

{ $project: { fullName: "$name", ageInMonths: { $multiply: ["age", 12] }, _id: 0 } }

medium

A. The _id field cannot be excluded.

B. Cannot use $multiply inside $project.

C. Field name "age" should be prefixed with $ to reference the field.

D. The field fullName cannot be renamed.

Solution

1. Step 1: Check field references in expressions

   In MongoDB aggregation, to reference a field inside expressions, prefix with $. Here, "age" is missing $.

2. Step 2: Validate other parts

   $multiply is valid in $project. Excluding _id is allowed. Renaming fields is allowed.

3. Final Answer:

   Field name "age" should be prefixed with $ to reference the field. -> Option C

4. Quick Check:

   Field references need $ prefix [OK]

Hint: Always prefix field names with $ inside expressions [OK]

Common Mistakes:

• Omitting $ before field names in expressions
• Thinking $multiply is invalid in $project
• Believing _id cannot be excluded

5. You have documents with fields firstName, lastName, and birthYear. Write a $project stage that outputs fullName as concatenation of first and last names separated by a space, and age calculated as 2024 minus birthYear. Which is the correct stage?

hard

A. { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } }

B. { $project: { fullName: "$firstName $lastName", age: 2024 - "$birthYear", _id: 0 } }

C. { $project: { fullName: { $concat: ["firstName", " ", "lastName"] }, age: { $subtract: [2024, "birthYear"] }, _id: 0 } }

D. { $project: { fullName: { $concat: ["$firstName", "$lastName"] }, age: { $subtract: ["birthYear", 2024] }, _id: 0 } }

Solution

1. Step 1: Build fullName using $concat with spaces

   Use $concat with "$firstName", a space string, and "$lastName" to join names with a space.

2. Step 2: Calculate age using $subtract

   Subtract birthYear field from 2024 using $subtract. Field references need $.

3. Step 3: Check each option

   { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } } correctly uses $concat with space and $subtract with field references. { $project: { fullName: "$firstName $lastName", age: 2024 - "$birthYear", _id: 0 } } uses invalid string concatenation and arithmetic. { $project: { fullName: { $concat: ["firstName", " ", "lastName"] }, age: { $subtract: [2024, "birthYear"] }, _id: 0 } } misses $ on fields. { $project: { fullName: { $concat: ["$firstName", "$lastName"] }, age: { $subtract: ["birthYear", 2024] }, _id: 0 } } misses space and reverses subtraction.

4. Final Answer:

   { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } } -> Option A

5. Quick Check:

   Use $concat with space and $subtract with $field [OK]

Hint: Use $concat with spaces and $subtract with $field for calculations [OK]

Common Mistakes:

• Forgetting $ before field names
• Concatenating fields without spaces
• Subtracting in wrong order
• Using string expressions instead of operators