MongoDB Aggregation $limit

Introduction

The $limit stage in MongoDB's aggregation framework is used to restrict the number of documents passed to the next stage in the pipeline. This is particularly useful when you only want to retrieve a subset of documents, such as the top results based on certain criteria.

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 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 3, "item": "apple", "quantity": 2, "price": 2 }
{ "_id": 4, "item": "orange", "quantity": 7, "price": 3 }
{ "_id": 5, "item": "banana", "quantity": 3, "price": 1 }
{ "_id": 6, "item": "grapes", "quantity": 4, "price": 4 }
{ "_id": 7, "item": "mango", "quantity": 8, "price": 5 }

Step 4: Using $limit

1. Basic Usage of $limit

To limit the number of documents returned in the aggregation pipeline, you can use the $limit stage.

For example, to retrieve only the first three documents from the sales collection:

db.sales.aggregate([
  { $limit: 3 }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 3, "item": "apple", "quantity": 2, "price": 2 }

2. Using $limit After Sorting

It is common to use $limit after sorting the results to get the top N documents based on a specific field. For example, if you want the top two items with the highest quantity sold:

db.sales.aggregate([
  { $sort: { quantity: -1 } }, // Sort in descending order of quantity
  { $limit: 2 } // Limit to the top 2
])

Output:

{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 7, "item": "mango", "quantity": 8, "price": 5 }

3. Combining $limit with Other Stages

You can combine $limit with other stages like $match and $group. For instance, if you want to find the top items sold, but only for those with a price greater than 1:

db.sales.aggregate([
  { $match: { price: { $gt: 1 } } }, // Filter items with price greater than 1
  { $sort: { quantity: -1 } }, // Sort by quantity in descending order
  { $limit: 3 } // Limit to the top 3
])

Output:

{ "_id": 7, "item": "mango", "quantity": 8, "price": 5 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 1, "item": "apple", "quantity": 5, "price": 2 }

Conclusion

You have learned how to use the $limit stage in MongoDB's aggregation framework to restrict the number of documents returned in a query. By combining $limit with other stages like $sort and $match, you can refine your queries to retrieve specific subsets of data.

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

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