MongoDB Aggregation $count

Introduction

The $count stage in MongoDB's aggregation framework is used to count the number of documents that pass through the pipeline and returns the count as a document with a specified field name. This stage is useful for obtaining a total count of documents based on certain criteria or conditions after applying various stages of the aggregation pipeline.

Prerequisites

• MongoDB installed and running on your machine.

• Access to mongosh and a populated database with sample data.

Step 1: Start mongosh

1. Open Terminal/Command Prompt:

   • Launch your terminal (macOS/Linux) or command prompt (Windows).

2. Start mongosh:

   • Type the following command and press Enter:

     mongosh

Step 2: Switch to the Database

Switch to the database where your collection is located.

1. Use a Database:

   use myNewDatabase

Step 3: Sample Data

Let’s assume you have a collection named sales with documents that look like this:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2, "category": "fruit" }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "category": "fruit" }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "category": "vegetable" }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "category": "fruit" }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5, "category": "vegetable" }

Step 4: Using $count

1. Basic Usage of $count

To count the total number of documents in the sales collection, you can use the $count stage at the end of your aggregation pipeline. For example:

db.sales.aggregate([
  { $count: "totalSales" } // Count all documents and label the output field as totalSales
])

Output:

{ "totalSales": 5 }

2. Counting After Filtering

You can also count the documents after applying certain conditions using $match followed by $count. For example, to count how many items are in the fruit category:

db.sales.aggregate([
  { $match: { category: "fruit" } }, // Filter for items in the fruit category
  { $count: "totalFruits" } // Count the filtered documents
])

Output:

{ "totalFruits": 3 }

3. Counting with Multiple Conditions

You can use $match to filter documents based on multiple conditions before counting. For example, to count the number of fruits with a quantity greater than 5:

db.sales.aggregate([
  { $match: { category: "fruit", quantity: { $gt: 5 } } }, // Filter for fruits with quantity greater than 5
  { $count: "highQuantityFruits" } // Count the filtered documents
])

Output:

{ "highQuantityFruits": 1 }

4. Combining $count with Other Stages

You can combine $count with other aggregation stages for more complex analyses. For example, if you want to group items by category and then count how many items are in each category, you can use $group followed by $count:

db.sales.aggregate([
  { $group: { _id: "$category", totalItems: { $sum: 1 } } }, // Group by category and count items
  { $count: "categoryCount" } // Count the number of unique categories
])

Output:

{ "categoryCount": 2 }

5. Counting Documents in a Sub-Pipeline

You can also use $count in a sub-pipeline if you want to count documents at a certain stage of your aggregation. For example, to count the number of documents after some transformations:

db.sales.aggregate([
  { $addFields: { totalPrice: { $multiply: ["$quantity", "$price"] } } }, // Add a totalPrice field
  { $count: "totalDocumentsAfterAddFields" } // Count the documents after the addFields stage
])

Output:

{ "totalDocumentsAfterAddFields": 5 }

Conclusion

You have learned how to use the $count stage in MongoDB's aggregation framework to count documents based on specified criteria and conditions. This stage is essential for analyzing data and obtaining aggregate counts that can be useful for reporting and data insights.

Feel free to reach out if you have any questions or need further guidance!

---