countDocuments method in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count all documents in the 'users' collection.

MongoDB

const count = await db.collection('users').[1]();

Drag options to blanks, or click blank then click option'

Afind

BcountDocuments

CinsertOne

DdeleteMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using find() instead of countDocuments() to count documents.

Trying to use insertOne() which adds documents, not counts them.

The countDocuments method returns the number of documents in the collection.

2fill in blank

medium

Complete the code to count documents where the field 'status' equals 'active'.

MongoDB

const activeCount = await db.collection('users').countDocuments({ [1]: 'active' });

Drag options to blanks, or click blank then click option'

Astatus

Bage

Cname

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong field name that does not exist in documents.

Leaving the query object empty when a filter is needed.

The query object specifies the field to filter documents. Here, we count documents with status equal to 'active'.

3fill in blank

hard

Fix the error in the code to correctly count documents with age greater than 30.

MongoDB

const count = await db.collection('users').countDocuments({ age: { [1]: 30 } });

Drag options to blanks, or click blank then click option'

A$gt

B$lt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt which means 'less than' instead of 'greater than'.

Using $eq which means 'equal to' instead of a comparison operator.

The MongoDB query operator $gt means 'greater than'. It filters documents where age is greater than 30.

4fill in blank

hard

Fill both blanks to count documents where 'score' is at least 80 and 'passed' is true.

MongoDB

const passedCount = await db.collection('exams').countDocuments({ score: { [1]: 80 }, [2]: true });

Drag options to blanks, or click blank then click option'

A$gte

Bpassed

Cscore

D$lte

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lte instead of $gte for the score comparison.

Using wrong field names for the boolean condition.

$gte means 'greater than or equal to', used to check if score is at least 80. The field passed is checked for true.

5fill in blank

hard

Fill all three blanks to count documents where 'category' is 'books', 'price' is less than 20, and 'inStock' is true.

MongoDB

const cheapBooksCount = await db.collection('products').countDocuments({ [1]: 'books', price: { [2]: 20 }, [3]: true });

Drag options to blanks, or click blank then click option'

Acategory

B$lt

CinStock

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up field names like using 'price' for category.

Using $gt instead of $lt for price comparison.

Using 'stock' instead of 'inStock' for availability.

The query filters documents where category equals 'books', price is less than 20 using $lt, and inStock is true.

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