countDocuments method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Counting documents with a simple filter

Given a MongoDB collection named users with documents containing an age field, what is the output of the following code?

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

Assume the collection has 5 documents where age is greater than 30.

MongoDB

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

A 0

B Error: countDocuments is not a function

C 5

D Number of documents in the collection

💡 Hint

The countDocuments method counts documents matching the filter.

The method countDocuments returns the number of documents that match the filter. Here, it counts documents where age is greater than 30, which is 5.

📝 Syntax

intermediate

Identify the syntax error in countDocuments usage

Which option contains a syntax error when using countDocuments to count documents where status is 'active'?

MongoDB

db.users.countDocuments({ status: 'active' })

A db.users.countDocuments({ status: 'active' })

B db.users.countDocuments({ status == 'active' })

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D db.users.countDocuments({ status: {$eq: 'active'} })

💡 Hint

Filters use colon : to assign values, not double equals ==.

Option B uses == inside the filter object, which is invalid syntax in MongoDB queries. Filters require key-value pairs with colon.

❓ optimization

advanced

Optimizing countDocuments with an index

You want to count documents where category is 'books' in a large collection. Which option will make countDocuments run faster?

A Create an index on the <code>category</code> field before running countDocuments.

B Use <code>countDocuments</code> without any index; MongoDB will optimize automatically.

C Use <code>estimatedDocumentCount</code> instead of <code>countDocuments</code> with a filter.

D Run <code>countDocuments</code> with an empty filter to count all documents.

💡 Hint

Indexes help MongoDB find matching documents faster.

Creating an index on the category field allows MongoDB to quickly locate documents matching the filter, speeding up countDocuments.

🧠 Conceptual

advanced

Difference between countDocuments and estimatedDocumentCount

Which statement correctly describes the difference between countDocuments and estimatedDocumentCount?

A Both methods always return the exact same count regardless of filters.

B <code>countDocuments</code> is faster than <code>estimatedDocumentCount</code> for large collections.

C <code>estimatedDocumentCount</code> requires a filter; <code>countDocuments</code> does not.

D <code>countDocuments</code> counts documents matching a filter; <code>estimatedDocumentCount</code> returns an approximate total count without a filter.

💡 Hint

One method can use filters, the other cannot.

countDocuments counts documents matching a filter and is accurate. estimatedDocumentCount returns a fast approximate count of all documents without filtering.

🔧 Debug

expert

Why does countDocuments return zero unexpectedly?

You run db.orders.countDocuments({ status: 'shipped' }) but it returns 0, even though you know there are shipped orders. Which option explains the most likely cause?

A The <code>status</code> field values are stored as uppercase strings like 'SHIPPED', so the filter does not match.

B The <code>countDocuments</code> method only counts documents inserted in the last 24 hours.

C The collection <code>orders</code> is empty.

D MongoDB does not support filtering with <code>countDocuments</code>.

💡 Hint

Filters are case sensitive and must match exactly.

If the status field values are uppercase but the filter uses lowercase, no documents match, so the count is zero.