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How to Use Custom Aggregation Expressions in MongoDB 4.4

July 28, 2020 for MongoDB

The upcoming release of MongoDB 4.4 makes it easier than ever to work with, transform, access, and make sense of your data. This release, the beta of which you can try right now , comes with a couple of new operators that make it possible to write custom functions to extend the MongoDB Query Language. This feature, called Custom Aggregation Expressions, allows you to write JavaScript functions that execute as part of an aggregation pipeline stage. These come in handy when you need to implement behavior that is not supported by the MongoDB Query Language by default.

The MongoDB Query Language has many operators, or functions, that allow you to manipulate and transform your data to fit your application’s use case. Operators such as $avg , $concat , and $filter make it easy for developers to query, manipulate, and transform their dataset directly at the database level versus having to write additional code and transforming the data elsewhere. While there are operators for almost anything you can think of, there are a few edge cases where a provided operator or series of operators won’t be sufficient, and that’s where custom aggregation expressions come in.

In this blog post, we’ll learn how we can extend the MongoDB Query Language to suit our needs by writing our own custom aggregation expressions using the new $function and $accumulator operators. Let’s dive in!

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