How to Retrieve the Last N Records in MongoDB?

Retrieve last N Records in MongoDB with sorting and limit

To retrieve the last N records in MongoDB, you typically need to sort the documents in reverse order (descending) by a timestamp or another field that indicates the order, and then limit the result to N documents. MongoDB does not directly support retrieving the last records, so sorting and limiting is the approach to use. Here’s a detailed explanation, syntax, example code, and how this query works.

Description:

In MongoDB, retrieving the last N records is commonly done by sorting the documents in descending order based on a specific field, such as a timestamp or an auto-incremented field. By doing this, you can use the limit function to fetch the latest N records in the sorted order. This approach is effective for retrieving recent entries in time-based data.

Syntax:

The syntax for getting the last N records is:

db.<collection>.find().sort({ <field>: -1 }).limit(N)

Explanation:

• <collection>: The name of the collection to query.
• <field>: The field to sort by, typically a date or timestamp.
• -1: Specifies descending order, so the latest records appear first.
• N: The number of documents you want to retrieve.

Example Code:

Here's an example that queries a posts collection to get the last 5 records based on a createdAt field (a timestamp indicating when each post was created).

Code:

// Query the 'posts' collection
db.posts.find()
  // Sort the documents by 'createdAt' in descending order
  .sort({ createdAt: -1 })
  // Limit the result to the last 5 records
  .limit(5)

Explanation:

• db.posts.find()
  This starts the query in the posts collection.
• .sort({ createdAt: -1 })
  This sorts the documents by the createdAt field in descending order, so the most recent records are listed first.
• .limit(5)
  Limits the result set to 5 documents, effectively retrieving the last 5 records based on the specified sorting.

By combining .sort() and .limit(), MongoDB will return the last 5 records based on the createdAt timestamp in descending order. This method is commonly used when you need the latest entries in time-series data.