Скачать или смотреть How to Use MongoDB $lookup to Limit Results to the Latest Episode for Each Show

Discover how to utilize MongoDB's `$lookup` to join collections effectively while limiting results to the latest episode for each show.
---
This video is based on the question https://stackoverflow.com/q/66593345/ asked by the user 'RussellHarrower' ( https://stackoverflow.com/u/275414/ ) and on the answer https://stackoverflow.com/a/66593495/ provided by the user 'Leo' ( https://stackoverflow.com/u/15119323/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: MongoDB $lookup creates array

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Introduction

If you’re working with MongoDB and need to join two collections—let’s say shows and episodes—using the $lookup function, you might encounter a common challenge. The key problem in this scenario is how to limit the results from the episodes collection to only the latest episode for each show, instead of getting all episodes associated with a show.

In this guide, we will walk you through an effective method to solve this problem by manipulating the $lookup syntax in MongoDB.

Understanding the Problem

Imagine you have two collections:

Shows: This collection contains information about various shows.

Episodes: This collection stores details about episodes, including which show they belong to.

When you execute a $lookup to join these collections, you might want to fetch only the most recent episode for each show based on its publication date (pubDate). However, a simple lookup may return all episodes related to a show. The goal is to limit the output to just one (the latest episode).

Initial Attempt

Your original query structure might look like this:

[[See Video to Reveal this Text or Code Snippet]]

Though this joins the collections correctly, it retrieves all episodes and does not limit them to the latest one.

Solution with the Enhanced $lookup

The solution lies in utilizing the second syntax of $lookup, incorporating a pipeline to filter and sort the results. Here’s how to do it:

Updated $lookup Syntax

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Pipeline Steps

Declaration:

let: {url: '$url'} declares a variable url that captures the current url value from the shows collection.

Matching:

$match uses $expr and $eq to find episodes where the show_b field matches the url variable.

Sorting:

The $sort stage arranges the episodes in a descending order based on pubDate, ensuring that the newest episodes come first.

Limiting:

The final stage, $limit: 1, restricts the output to only the first episode after sorting—effectively giving you the latest episode for each show.

Conclusion

By using this enhanced $lookup syntax in MongoDB, you can efficiently join your shows and episodes collections and ensure you only fetch the latest episode per show. This approach not only improves the performance of your queries but also ensures the relevance of your data output.

Feel free to implement this solution in your MongoDB queries, and streamline your database interactions!