$project stage for shaping output in MongoDB - Mini Project: Build & Apply

$project stage for shaping output

📖 Scenario: You work at a small bookstore that keeps its inventory in a MongoDB collection. Each book document has details like title, author, price, and stock count.You want to create a report that only shows the book title and price, hiding other details.

🎯 Goal: Build a MongoDB aggregation pipeline using the $project stage to shape the output documents to include only the title and price fields.

📋 What You'll Learn

Create a collection named books with 3 book documents having fields title, author, price, and stock.

Define a variable projection that specifies the $project stage to include only title and price.

Write an aggregation query on books using the projection variable.

Complete the pipeline so the output documents only show title and price.

💡 Why This Matters

🌍 Real World

Shaping output with $project is common when you want to show only relevant data to users or reports, hiding sensitive or unnecessary fields.

💼 Career

Many jobs require working with MongoDB aggregations to optimize data retrieval and presentation, making $project a key skill.

1

Create the books collection with 3 book documents

Create a variable called books and assign it an array with these exact 3 documents:
{ title: "The Hobbit", author: "J.R.R. Tolkien", price: 15, stock: 5 },
{ title: "1984", author: "George Orwell", price: 12, stock: 8 },
{ title: "To Kill a Mockingbird", author: "Harper Lee", price: 10, stock: 3 }

MongoDB

const books = [
  // Your code here
];

Hint

Use an array of objects with the exact field names and values.

2

Define the projection variable for the $project stage

Create a variable called projection and assign it an object with a $project field that includes only title and price set to 1.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", price: 15, stock: 5 },
  { title: "1984", author: "George Orwell", price: 12, stock: 8 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", price: 10, stock: 3 }
];

// Your code here

Hint

Use $project with title: 1 and price: 1 to include only those fields.

3

Write the aggregation query using the projection variable

Create a variable called result and assign it the result of calling books.aggregate with an array containing the projection variable.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", price: 15, stock: 5 },
  { title: "1984", author: "George Orwell", price: 12, stock: 8 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", price: 10, stock: 3 }
];

const projection = { $project: { title: 1, price: 1 } };

// Your code here

Hint

Use books.aggregate with an array containing projection.

4

Complete the pipeline to output only title and price

Add a final line to the code that assigns resultArray to the array form of result by calling toArray().

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", price: 15, stock: 5 },
  { title: "1984", author: "George Orwell", price: 12, stock: 8 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", price: 10, stock: 3 }
];

const projection = { $project: { title: 1, price: 1 } };

const result = books.aggregate([projection]);

// Your code here

Hint

Call toArray() on result to get the final array.

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