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' })

C )} 'evitca' :sutats {(stnemucoDtnuoc.sresu.bd

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.

Practice

1.

What does the countDocuments method do in MongoDB?

easy

A. Counts how many documents match a given filter

B. Deletes documents matching a filter

C. Updates documents matching a filter

D. Returns all documents in a collection

Solution

1. Step 1: Understand the purpose of countDocuments

   The countDocuments method is used to count documents that match a filter in a collection.

2. Step 2: Compare with other operations

   Deleting, updating, or returning documents are different operations and not related to counting.

3. Final Answer:

   Counts how many documents match a given filter -> Option A

4. Quick Check:

   countDocuments = count matching documents [OK]

Hint: Remember: countDocuments counts, not modifies data [OK]

Common Mistakes:

• Confusing countDocuments with delete or update methods
• Thinking it returns the documents instead of a count
• Assuming it counts all documents without a filter

2.

Which of the following is the correct syntax to count documents with status equal to "active" in a collection named users?

?

easy

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

B. db.users.count({ status: active })

C. db.users.find({ status: "active" }).countDocuments({ status: "active" })

D. db.users.countDocuments("status = active")

Solution

1. Step 1: Identify correct method usage

   The countDocuments method is called on the collection with a filter object inside parentheses.

2. Step 2: Check filter format

   The filter must be an object like { status: "active" }, not a string or chained after find().

3. Final Answer:

   db.users.countDocuments({ status: "active" }) -> Option A

4. Quick Check:

   Correct syntax uses countDocuments(filter) [OK]

Hint: Use countDocuments with a filter object inside parentheses [OK]

Common Mistakes:

• Using deprecated count() method
• Passing filter as a string instead of object
• Calling countDocuments after find()

3.

Given the collection orders with documents:

[{ "status": "shipped" }, { "status": "pending" }, { "status": "shipped" }]

What will db.orders.countDocuments({ status: "shipped" }) return?

medium

A. 1

B. 0

C. 2

D. 3

Solution

1. Step 1: Identify documents matching the filter

   Documents with status: "shipped" are the first and third documents.

2. Step 2: Count matching documents

   There are 2 such documents in total.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Count of shipped orders = 2 [OK]

Hint: Count documents matching filter exactly [OK]

Common Mistakes:

• Counting all documents instead of filtered ones
• Misreading the status values
• Assuming countDocuments returns documents, not count

4.

What is wrong with this code snippet?

const count = db.products.countDocuments("category: 'books'");

It aims to count documents where category is "books".

medium

A. Missing await keyword for asynchronous call

B. The filter is passed as a string instead of an object

C. countDocuments cannot be used on the products collection

D. The method name should be count, not countDocuments

Solution

1. Step 1: Check filter argument type

   The filter must be an object like { category: 'books' }, not a string.

2. Step 2: Confirm method usage

   countDocuments is valid on collections and accepts an object filter.

3. Final Answer:

   The filter is passed as a string instead of an object -> Option B

4. Quick Check:

   Filter must be an object, not a string [OK]

Hint: Filters are objects, not strings in countDocuments [OK]

Common Mistakes:

• Passing filter as a string
• Confusing countDocuments with count
• Ignoring async/await in some environments (not always error)

5.

You want to count how many users have either age greater than 30 or status equal to "active". Which query correctly uses countDocuments to do this?

hard

A. db.users.countDocuments({ age > 30 || status == "active" })

B. db.users.countDocuments({ age: { $gt: 30 }, status: "active" })

C. db.users.countDocuments({ $and: [ { age: { $gt: 30 } }, { status: "active" } ] })

D. db.users.countDocuments({ $or: [ { age: { $gt: 30 } }, { status: "active" } ] })

Solution

1. Step 1: Understand the filter logic

   We want documents where age is greater than 30 OR status is "active".

2. Step 2: Use correct MongoDB query syntax

   The $or operator takes an array of conditions to match either one.

3. Step 3: Check each option

   The option using { $or: [ { age: { $gt: 30 } }, { status: "active" } ] } is correct. The option with comma-separated conditions uses implicit AND, the one with $and uses explicit AND, and the last uses invalid syntax.

4. Final Answer:

   db.users.countDocuments({ $or: [ { age: { $gt: 30 } }, { status: "active" } ] }) -> Option D

5. Quick Check:

   Use $or with array for OR conditions [OK]

Hint: Use $or with array for OR conditions in filters [OK]

Common Mistakes:

• Using implicit AND instead of OR
• Writing invalid filter syntax
• Confusing $and and $or operators