$count accumulator in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Counting documents with $count accumulator

Given a MongoDB collection orders with documents representing customer orders, what will be the output of this aggregation pipeline?

[{ $match: { status: "completed" } }, { $count: "totalCompleted" }]

MongoDB

[{ $match: { status: "completed" } }, { $count: "totalCompleted" }]

A [ { "totalCompleted": 5 } ]

B [ { "totalCompleted": 0 } ]

C [ { "count": 5 } ]

D []

💡 Hint

The $count stage returns a document with the field name you specify and the count of documents passed to it.

The pipeline first filters documents with status 'completed'. Then $count returns a single document with the field 'totalCompleted' and the number of matched documents. If there are 5 such documents, the output is [{ "totalCompleted": 5 }].

🧠 Conceptual

intermediate

Understanding $count accumulator behavior

What does the $count accumulator stage do in a MongoDB aggregation pipeline?

A Counts the number of fields in each document and outputs the count per document.

B Counts the number of documents input to it and outputs a single document with the count.

C Counts the number of distinct values of a specified field.

D Counts the total size in bytes of all documents passed to it.

💡 Hint

Think about what $count outputs after processing all documents.

$count counts how many documents it receives and outputs exactly one document with the count under the specified field name.

📝 Syntax

advanced

Identify the syntax error in $count usage

Which of the following aggregation pipelines will cause a syntax error due to incorrect $count usage?

A [{ $count: "total" }]

B [{ $match: { status: "active" } }, { $count: "activeCount" }]

C [{ $group: { _id: null, count: { $sum: 1 } } }]

D [{ $count: { total: true } }]

💡 Hint

$count expects a string as its argument, not an object.

The $count stage requires a string specifying the output field name. Option D passes an object, which is invalid syntax and causes an error.

❓ optimization

advanced

Optimizing count queries with $count

You want to count documents with a specific condition in a large MongoDB collection. Which pipeline is more efficient?

A [{ $group: { _id: "$status", count: { $sum: 1 } } }, { $match: { _id: "pending" } }]

B [{ $project: { status: 1 } }, { $count: "count" }]

C [{ $match: { status: "pending" } }, { $count: "pendingCount" }]

D [{ $match: { status: { $exists: true } } }, { $count: "count" }]

💡 Hint

Filtering early reduces the number of documents processed by $count.

Option C filters documents first with $match, then counts only those matching. This reduces workload and is more efficient than grouping all documents or counting unrelated documents.

🔧 Debug

expert

Why does this $count pipeline return an empty array?

Consider this aggregation pipeline:

[{ $match: { age: { $gt: 100 } } }, { $count: "oldPeople" }]

It returns an empty array []. Why?

A No documents match the condition, so $count outputs no documents.

B $count always returns an empty array if used after $match.

C The field name in $count must be _id, otherwise it returns empty.

D The $match stage is invalid and filters out all documents.

💡 Hint

Think about what happens if no documents pass the $match filter.

If no documents match the $match condition, $count receives zero documents and returns an empty array instead of a document with count 0.