MongoDB Shell (mongosh) basics - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with a specific field value

Given a collection users with documents containing age and name, what is the output of this query?

db.users.find({ age: { $gt: 30 } }).toArray()

Assume the collection has:
{ name: 'Alice', age: 25 }
{ name: 'Bob', age: 35 }
{ name: 'Carol', age: 40 }

MongoDB

db.users.find({ age: { $gt: 30 } }).toArray()

A [{ name: 'Alice', age: 25 }, { name: 'Bob', age: 35 }, { name: 'Carol', age: 40 }]

B [{ name: 'Alice', age: 25 }]

C [{ name: 'Bob', age: 35 }, { name: 'Carol', age: 40 }]

D []

💡 Hint

Look for documents where age is greater than 30.

The query finds all users with age greater than 30, so Bob and Carol match.

📝 Syntax

intermediate

Identify the syntax error in the query

Which option contains a syntax error when trying to find documents where status is 'active'?

MongoDB

db.orders.find({ status: 'active' })

A db.orders.find({ status: 'active' })

B db.orders.find({ status = 'active' })

C db.orders.find({ 'status': 'active' })

D db.orders.find({ status: "active" })

💡 Hint

Check the use of colon and equal sign in object syntax.

In MongoDB queries, the filter object uses colon ':' to assign values, not '='.

❓ optimization

advanced

Optimize query to return only specific fields

You want to find all documents in products collection but only return the name and price fields. Which query is correct?

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

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

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

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

💡 Hint

Use projection to include fields by setting them to 1.

Projection with 1 includes fields; 0 excludes. Mixing 1 and 0 (except _id) is invalid.

🧠 Conceptual

advanced

Understanding the effect of update operators

What will be the result of this update operation?

db.inventory.updateOne({ item: 'book' }, { $set: { qty: 50 }, $inc: { sold: 5 } })

Assuming the document before update is:
{ item: 'book', qty: 20, sold: 10 }

A { item: 'book', qty: 50, sold: 15 }

B { item: 'book', qty: 70, sold: 15 }

C { item: 'book', qty: 50, sold: 5 }

D { item: 'book', qty: 20, sold: 15 }

💡 Hint

Remember $set replaces the field value; $inc adds to the existing value.

$set changes qty to 50; $inc adds 5 to sold (10 + 5 = 15).

🔧 Debug

expert

Identify the cause of the error in aggregation pipeline

This aggregation pipeline causes an error:

db.sales.aggregate([ { $match: { status: 'A' } }, { $group: { _id: '$cust_id', total: { $sum: '$amount' } } }, { $sort: { total: -1 } } ])

What is the most likely cause of the error?

A The $match stage is missing a required field

B The $sort stage must come before $group stage

C Field names in $group stage must not start with '$'

D The $sum operator requires numeric values but 'amount' contains strings

💡 Hint

Check data types for aggregation operators.

$sum requires numeric values; if 'amount' has strings, aggregation fails.