Projection for selecting fields 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, which query will return only the name and email fields for all users?

MongoDB

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

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

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

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

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

💡 Hint

Remember to exclude the _id field if you don't want it in the output.

Option C includes name and email fields and excludes _id. Option C includes _id by default. Options A and D exclude or mix fields incorrectly.

❓ query_result

intermediate

Projection excluding a field

Which MongoDB query will return all fields except the password field from the accounts collection?

MongoDB

db.accounts.find({}, {password: 0})

A db.accounts.find({}, {password: 0})

B db.accounts.find({}, {password: false})

C db.accounts.find({}, {password: 1})

D db.accounts.find({}, {password: null})

💡 Hint

Use 0 to exclude a field in projection.

Option A correctly excludes the password field. Option A includes only password. Options C and D use invalid values for projection.

📝 Syntax

advanced

Identify the syntax error in projection

Which option contains a syntax error in the MongoDB projection part of the query?

MongoDB

db.products.find({}, {name: 1, price: 'yes'})

A db.products.find({}, {name: 1, price: 'yes'})

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

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

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

💡 Hint

Projection values must be 0 or 1, not strings.

Option A uses a string 'yes' instead of 0 or 1, causing a syntax error. Other options use valid numeric values.

❓ query_result

advanced

Projection with nested fields

Given documents with a nested address field containing city and zip, which query returns only the name and address.city fields?

MongoDB

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

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

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

C db.customers.find({}, {name: 1, 'address.zip': 1})

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

💡 Hint

Use dot notation to project nested fields.

Option D correctly projects name and nested address.city fields. Option D includes whole address. Option D projects wrong nested field. Option D excludes address.

🧠 Conceptual

expert

Understanding projection behavior with _id field

In MongoDB, when you specify a projection like {name: 1} without mentioning _id, what will be the output regarding the _id field?

A The <code>_id</code> field is excluded by default in the output.

B The <code>_id</code> field is included by default in the output.

C The query will throw an error because <code>_id</code> is not specified.

D The <code>_id</code> field will be included only if explicitly set to 1.

💡 Hint

Think about MongoDB's default behavior for _id in projections.

MongoDB includes the _id field by default unless explicitly excluded with _id: 0. So option B is correct.