Projection for selecting fields in MongoDB - Mini Project: Build & Apply

Projection for Selecting Fields in MongoDB

📖 Scenario: You are managing a small online bookstore database. The books collection contains documents with details about each book, including title, author, year published, genre, and price.You want to learn how to retrieve only specific information from the database to make your queries faster and easier to read.

🎯 Goal: Build a MongoDB query that uses projection to select only the title and author fields from the books collection.

📋 What You'll Learn

Create a books collection with 3 book documents having fields: title, author, year, genre, and price.

Create a variable called projection that selects only the title and author fields.

Write a MongoDB query using find() on books with the projection variable.

Add a final line to execute the query and store the results in a variable called selectedBooks.

💡 Why This Matters

🌍 Real World

Selecting only needed fields from a database makes queries faster and reduces data transfer, which is important for web apps and APIs.

💼 Career

Database developers and backend engineers often use projection to optimize queries and improve application performance.

1

Create the books collection with sample documents

Create a variable called books and assign it an array with exactly these 3 documents:
{ title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy", price: 15 },
{ title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian", price: 12 },
{ title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction", price: 10 }.

MongoDB

const books = [
  // Your code here
];

Hint

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

2

Create the projection variable to select fields

Create a variable called projection and set it to an object that includes title and author fields with value 1 to include them in the query result.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy", price: 15 },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian", price: 12 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction", price: 10 }
];

// Your code here

Hint

Use { title: 1, author: 1 } to include only these fields.

3

Write the MongoDB find() query with projection

Write a query using db.books.find() with an empty filter object {} and the projection variable as the second argument. Assign this query to a variable called query.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy", price: 15 },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian", price: 12 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction", price: 10 }
];

const projection = { title: 1, author: 1 };

// Your code here

Hint

Use db.books.find({}, projection) to select only the fields in projection.

4

Execute the query and store the results

Add a line to execute the query by calling toArray() and assign the result to a variable called selectedBooks.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy", price: 15 },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian", price: 12 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction", price: 10 }
];

const projection = { title: 1, author: 1 };

const query = db.books.find({}, projection);

// Your code here

Hint

Use query.toArray() to get the results as an array.

Practice

1. What does projection do in a MongoDB query?

easy

A. It selects which fields to include or exclude in the query result.

B. It sorts the documents in the collection.

C. It updates the documents in the collection.

D. It deletes documents from the collection.

Solution

1. Step 1: Understand the purpose of projection

   Projection in MongoDB is used to specify which fields to include or exclude in the output of a query.

2. Step 2: Differentiate projection from other operations

   Sorting, updating, and deleting are different operations and not related to projection.

3. Final Answer:

   It selects which fields to include or exclude in the query result. -> Option A

4. Quick Check:

   Projection = Select fields [OK]

Hint: Projection picks fields to show, not to sort or update [OK]

Common Mistakes:

• Confusing projection with sorting
• Thinking projection updates data
• Assuming projection deletes documents

2. Which of the following is the correct syntax to include only the name and age fields in a MongoDB find query?

easy

A. { name: 1, age: 1 }

B. { name: true, age: true }

C. { name: 'include', age: 'include' }

D. { name: 0, age: 0 }

Solution

1. Step 1: Recall projection syntax

   In MongoDB, to include fields, you set them to 1 in the projection document.

2. Step 2: Check each option

   { name: 1, age: 1 } uses 1 correctly; { name: true, age: true } uses true which is valid in MongoDB; { name: 'include', age: 'include' } uses strings which is invalid; { name: 0, age: 0 } excludes fields instead of including.

3. Final Answer:

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

4. Quick Check:

   Include fields = 1 [OK]

Hint: Use 1 to include fields, not strings [OK]

Common Mistakes:

• Using strings like 'include'
• Using 0 to include fields

3. Given the collection documents:

{ _id: 1, name: "Alice", age: 25, city: "NY" }

{ _id: 2, name: "Bob", age: 30, city: "LA" }

What will be the result of the query db.collection.find({}, { name: 1, city: 1 })?

medium

A. [{ _id: 1, age: 25 }, { _id: 2, age: 30 }]

B. [{ name: "Alice", city: "NY" }, { name: "Bob", city: "LA" }]

C. [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }]

D. [{ name: "Alice", age: 25, city: "NY" }, { name: "Bob", age: 30, city: "LA" }]

Solution

1. Step 1: Understand default _id behavior

   By default, MongoDB includes the _id field unless explicitly excluded.

2. Step 2: Check projection fields

   The query includes name and city fields with 1, so these fields plus _id will appear.

3. Final Answer:

   [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }] -> Option C

4. Quick Check:

   Projection includes fields + _id by default [OK]

Hint: Projection includes _id unless set to 0 [OK]

Common Mistakes:

• Assuming _id is excluded by default
• Expecting age field in result
• Confusing included and excluded fields

4. What is wrong with this MongoDB query to exclude the password field?

db.users.find({}, { password: 1 })

medium

A. The syntax for projection is incorrect.

B. It includes the password field instead of excluding it.

C. You cannot exclude fields in MongoDB projection.

D. The query will return an error because password is a reserved word.

Solution

1. Step 1: Understand projection values

   Setting a field to 1 includes it; setting to 0 excludes it.

2. Step 2: Analyze the query

   The query sets password: 1, so it includes the password field (and _id), not excludes it.

3. Final Answer:

   It includes the password field instead of excluding it. -> Option B

4. Quick Check:

   Use 0 to exclude fields [OK]

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

Common Mistakes:

• Using 1 to exclude fields
• Thinking password is reserved word
• Believing exclusion is not possible

5. You want to fetch documents from a collection but exclude the _id and password fields while including all others. Which projection is correct?

hard

A. { _id: 1, password: 1 }

B. { _id: 1, password: 0 }

C. { _id: 0, password: 1 }

D. { _id: 0, password: 0 }

Solution

1. Step 1: Understand exclusion in projection

   Setting fields to 0 excludes them from the result.

2. Step 2: Apply exclusion to _id and password

   To exclude both _id and password, set both to 0 in the projection.

3. Final Answer:

   { _id: 0, password: 0 } -> Option D

4. Quick Check:

   Exclude fields with 0, include others by default [OK]

Hint: Set unwanted fields to 0, others stay included [OK]

Common Mistakes:

• Mixing inclusion and exclusion in projection
• Forgetting to exclude _id explicitly
• Setting excluded fields to 1