Projection for reducing data transfer in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Projection to include specific fields

Given a MongoDB collection users with documents containing name, age, and email, what will be the output of this query?

db.users.find({}, {name: 1, email: 1, _id: 0})

Assume the collection has:

{"name": "Alice", "age": 30, "email": "[e-mail]"}

{"name": "Bob", "age": 25, "email": "[e-mail]"}

MongoDB

db.users.find({}, {name: 1, email: 1, _id: 0})

A [{"name": "Alice", "email": "[e-mail]"}, {"name": "Bob", "email": "[e-mail]"}]

B [{"name": "Alice", "age": 30, "email": "[e-mail]"}, {"name": "Bob", "age": 25, "email": "[e-mail]"}]

C [{"age": 30, "email": "[e-mail]"}, {"age": 25, "email": "[e-mail]"}]

D [{"name": "Alice"}, {"name": "Bob"}]

💡 Hint

Projection includes only the fields set to 1 and excludes _id when set to 0.

The projection specifies to include only 'name' and 'email' fields and exclude '_id'. So the output documents contain only those fields.

❓ query_result

intermediate

Projection excluding fields

What will be the result of this MongoDB query?

db.products.find({}, {description: 0, _id: 0})

Given documents:

{"name": "Pen", "price": 1.5, "description": "Blue ink pen"}

{"name": "Notebook", "price": 3.0, "description": "200 pages"}

MongoDB

db.products.find({}, {description: 0, _id: 0})

A [{"price": 1.5}, {"price": 3.0}]

B [{"name": "Pen", "price": 1.5, "description": "Blue ink pen"}, {"name": "Notebook", "price": 3.0, "description": "200 pages"}]

C [{"name": "Pen", "price": 1.5}, {"name": "Notebook", "price": 3.0}]

D [{"name": "Pen"}, {"name": "Notebook"}]

💡 Hint

Setting a field to 0 excludes it from the result.

The projection excludes 'description' and '_id', so only 'name' and 'price' remain in the output.

📝 Syntax

advanced

Identify the invalid projection syntax

Which of the following MongoDB queries has invalid projection syntax and will cause an error?

A db.orders.find({}, {item: true, quantity: true})

B db.orders.find({}, {item: 1, quantity: 1, _id: 0})

C db.orders.find({}, {item: 1, quantity: 0})

D db.orders.find({}, {item: 1, quantity: 1, _id: 1, price: 0})

💡 Hint

You cannot mix inclusion (1) and exclusion (0) in the same projection except for _id.

Option D mixes inclusion (item, quantity, _id) and exclusion (price), which is invalid and causes an error.

❓ optimization

advanced

Optimizing data transfer with projection

You have a large collection logs with documents containing many fields. You only need the timestamp and level fields for your report. Which query best reduces data transfer?

A db.logs.find({}, {timestamp: 1, level: 1, _id: 0})

B db.logs.find({}, {timestamp: 1, level: 1})

C db.logs.find({}, {})

D db.logs.find({}, {timestamp: 0, level: 0})

💡 Hint

Exclude _id if you don't need it to reduce data size further.

Option A includes only needed fields and excludes _id, minimizing data sent over the network.

🧠 Conceptual

expert

Understanding projection behavior with nested fields

Consider documents in employees collection with nested address field:

{"name": "John", "address": {"city": "NY", "zip": "10001"}, "age": 28}

What will be the output of this query?

db.employees.find({}, {"address.city": 1, _id: 0})

MongoDB

db.employees.find({}, {"address.city": 1, _id: 0})

A [{"address": {"city": "NY", "zip": "10001"}}]

B [{"address": {"city": "NY"}}]

C [{"address.city": "NY"}]

D [{"name": "John", "address": {"city": "NY"}}]

💡 Hint

Projection on nested fields returns the parent object with only the specified nested fields.

The query returns the 'address' object but only with the 'city' field included, excluding other fields and _id.

Practice

1. What is the main purpose of using projection in a MongoDB query?

easy

A. To select only specific fields from documents to reduce data transfer

B. To sort the documents in ascending order

C. To delete documents from the collection

D. To update multiple documents at once

Solution

1. Step 1: Understand projection concept

   Projection in MongoDB is used to specify which fields to include or exclude in the query result.

2. Step 2: Identify the purpose of projection

   Projection reduces the amount of data sent by selecting only needed fields, improving speed and efficiency.

3. Final Answer:

   To select only specific fields from documents to reduce data transfer -> Option A

4. Quick Check:

   Projection = Select fields only [OK]

Hint: Projection picks fields to reduce data size [OK]

Common Mistakes:

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

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

easy

A. db.collection.find({}, {name: 1, age: 1})

B. db.collection.find({name: 1, age: 1})

C. db.collection.find({}, {name: 0, age: 0})

D. db.collection.find({name: 0, age: 0})

Solution

1. Step 1: Recall find() method syntax

   The first argument is the filter (empty {} means all documents), the second is the projection specifying fields to include or exclude.

2. Step 2: Identify correct projection for inclusion

   Using 1 includes fields; so {name: 1, age: 1} includes only those fields.

3. Final Answer:

   db.collection.find({}, {name: 1, age: 1}) -> Option A

4. Quick Check:

   Include fields = 1 in second argument [OK]

Hint: Use 1 to include fields in projection [OK]

Common Mistakes:

• Putting projection in the first argument
• Using 0 to include fields
• Omitting the empty filter {}

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 output of this query?
db.collection.find({}, {name: 1, _id: 0})

medium

A. [{"_id": 1, "name": "Alice"}, {"_id": 2, "name": "Bob"}]

B. [{"age": 25, "city": "NY"}, {"age": 30, "city": "LA"}]

C. [{"name": "Alice"}, {"name": "Bob"}]

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

Solution

1. Step 1: Analyze projection fields

   The projection includes only name with 1 and excludes _id with 0, so only the name field appears without _id.

2. Step 2: Determine output documents

   Each document will have only the name field, no _id or other fields.

3. Final Answer:

   [{"name": "Alice"}, {"name": "Bob"}] -> Option C

4. Quick Check:

   Projection includes name only, excludes _id [OK]

Hint: Set _id:0 to exclude _id field [OK]

Common Mistakes:

• Forgetting _id is included by default
• Expecting other fields to appear
• Using 0 to include fields

4. Identify the error in this MongoDB query that tries to exclude the password field:
db.users.find({}, {password: 1, email: 0})

medium

A. Projection must include _id explicitly

B. The filter argument is missing

C. The query is correct and will work as expected

D. You cannot mix including and excluding fields in projection except for _id

Solution

1. Step 1: Understand projection rules

   MongoDB does not allow mixing inclusion (1) and exclusion (0) in the same projection except for the _id field.

2. Step 2: Analyze the given projection

   The projection tries to include password (1) and exclude email (0) together, which is invalid.

3. Final Answer:

   You cannot mix including and excluding fields in projection except for _id -> Option D

4. Quick Check:

   Mixing 1 and 0 in projection = error [OK]

Hint: Include or exclude fields, not both (except _id) [OK]

Common Mistakes:

• Mixing 1 and 0 in projection
• Assuming _id must be included explicitly
• Ignoring filter argument is optional

5. You want to retrieve only the title and author fields from a large books collection but exclude the _id field to reduce data size. Which query correctly achieves this and why?

hard

A. db.books.find({}, {title: 0, author: 0, _id: 1}) because excluding fields reduces data

B. db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id

C. db.books.find({}, {title: 1, author: 1}) because _id is excluded by default

D. db.books.find({}, {title: 1, author: 0, _id: 0}) because mixing inclusion and exclusion is allowed

Solution

1. Step 1: Determine fields to include and exclude

   You want to include title and author fields and exclude _id to reduce data size.

2. Step 2: Apply correct projection syntax

   Including fields uses 1, excluding _id uses 0. Mixing inclusion and exclusion (except _id) is not allowed.

3. Step 3: Evaluate options

   db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id correctly includes title and author with 1 and excludes _id with 0. Others either exclude needed fields or mix inclusion/exclusion incorrectly.

4. Final Answer:

   db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id -> Option B

5. Quick Check:

   Include fields with 1, exclude _id with 0 [OK]

Hint: Include needed fields with 1, exclude _id with 0 [OK]

Common Mistakes:

• Assuming _id is excluded by default
• Mixing inclusion and exclusion fields
• Excluding needed fields by mistake