findOne method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find the first user named 'Alice'

Given a collection users with documents containing name and age, what will this query return?

db.users.findOne({ name: 'Alice' })

MongoDB

db.users.findOne({ name: 'Alice' })

A All documents where the name is 'Alice'

B The first document where the name is 'Alice', including all fields

C An array of documents with name 'Alice'

D An error because findOne requires a projection

💡 Hint

Remember, findOne returns only one document, not multiple.

The findOne method returns the first document matching the query filter. It does not return multiple documents or an array.

📝 Syntax

intermediate

Identify the syntax error in this findOne query

Which option shows the correct syntax for finding a user with age 30 using findOne?

MongoDB

db.users.findOne({ age: 30 })

A db.users.findOne({ age: 30 })

B db.users.findOne(age: 30)

C db.users.findOne({age == 30})

D db.users.findOne({ 'age': '30' })

💡 Hint

The query filter must be an object with key-value pairs.

Option A uses the correct object syntax with key and value. Option A misses braces, A uses invalid comparison operator, D uses string instead of number for age.

❓ query_result

advanced

What does this findOne query return with projection?

Given documents with fields name, age, and city, what will this query return?

db.users.findOne({ name: 'Bob' }, { age: 1, _id: 0 })

MongoDB

db.users.findOne({ name: 'Bob' }, { age: 1, _id: 0 })

A An error because projection keys must be strings

B A document with all fields of the first user named 'Bob'

C A document with age and _id fields of the first user named 'Bob'

D A document with only the age field of the first user named 'Bob', excluding _id

💡 Hint

Projection controls which fields are included or excluded in the result.

The projection includes age (1) and excludes _id (0), so only age is returned without _id.

🔧 Debug

advanced

Why does this findOne query return null?

Consider this query:

db.users.findOne({ age: { $gt: 50 } })

It returns null. What is the most likely reason?

A The $gt operator is invalid in findOne

B The query syntax is incorrect and causes an error

C No document in the collection has age greater than 50

D findOne always returns null if multiple documents match

💡 Hint

Think about what null means in findOne results.

findOne returns null if no document matches the query. $gt is valid syntax and findOne returns the first match if any.

🧠 Conceptual

expert

Understanding findOne behavior with multiple matches

If multiple documents match the filter in a findOne query, which document is returned?

A The first document according to the natural order in the collection

B The document with the highest _id value

C A random document from the matches

D An error indicating multiple matches

💡 Hint

Think about how MongoDB stores documents internally.

findOne returns the first document it finds in the natural order (insertion order) that matches the filter.