Subset pattern for large documents in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find subset fields in a large document

Given a collection users where each document contains many fields, which query returns only the name and email fields for all users?

MongoDB

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

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

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

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

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

💡 Hint

Use projection to include only the fields you want and exclude _id if not needed.

Option C correctly includes only 'name' and 'email' fields and excludes '_id'. Option C includes '_id' by default. Option C excludes 'name' and 'email' which is opposite of the goal. Option C mixes inclusion and exclusion which is invalid.

🧠 Conceptual

intermediate

Why use subset pattern for large documents?

What is the main benefit of using the subset pattern when querying large documents in MongoDB?

A It automatically indexes all fields in the document.

B It increases the size of documents stored in the database.

C It duplicates data to improve query speed.

D It reduces the amount of data transferred over the network.

💡 Hint

Think about network efficiency and response size.

Using subset pattern returns only needed fields, reducing data sent over the network and improving performance. Other options are incorrect or unrelated.

📝 Syntax

advanced

Correct projection syntax for nested fields

Which query correctly returns only the name and the nested field address.city from documents in the customers collection?

MongoDB

db.customers.find({}, {name: 1, 'address.city': 1, _id: 0})

A db.customers.find({}, {name: 1, 'address.city': 1, _id: 0})

B db.customers.find({}, {name: 1, address.city: 1, _id: 0})

C db.customers.find({}, {name: 1, address: 1, city: 1, _id: 0})

D db.customers.find({}, {name: 1, address: {city: 1}, _id: 0})

💡 Hint

Use quotes for nested field names with dots.

Option A uses correct syntax with quotes around 'address.city'. Option A is invalid syntax. Option A tries to nest projection which is not supported. Option A incorrectly projects 'city' as a top-level field.

❓ optimization

advanced

Optimizing queries with subset pattern

You have a collection with large documents. You only need to display the title and summary fields in your app. Which approach optimizes query performance?

A Use projection to return only <code>title</code> and <code>summary</code> fields.

B Return full documents and filter fields in the application code.

C Create a new collection with only <code>title</code> and <code>summary</code> fields.

D Use aggregation pipeline without projection.

💡 Hint

Think about minimizing data transfer and processing.

Using projection reduces data sent and speeds up queries. Filtering in app wastes bandwidth. Creating new collection duplicates data and adds maintenance. Aggregation without projection returns full documents.

🔧 Debug

expert

Identify the error in this subset query

What error will this MongoDB query produce?

db.products.find({}, {name: 1, price: 1, quantity: 0})

A No error, returns name and price fields only

B MongoError: Cannot mix inclusion and exclusion in projection

C SyntaxError: Unexpected token in projection

D TypeError: Invalid projection format

💡 Hint

In projection, you cannot mix including and excluding fields except for _id.

MongoDB projection rules forbid mixing inclusion (1) and exclusion (0) except for _id. Here, 'quantity: 0' conflicts with 'name: 1' and 'price: 1', causing an error.