The aggregation framework is one my favorite tools in MongoDB. Its a clean way to take a set of data and run it through a pipeline of steps to modify, analyze, and process data.

At MongoDB World, one of the features we talked about that is coming in MongoDB 3.2 is $lookup. $lookup is an aggregation stage that lets you run a query on a different collection and put the results into a document in your pipeline. This is a pretty powerful feature that we’ll talk more about in a later post.

In order to make writing $lookup a bit cleaner, we’ve done some work to make adding aggregation stages easier. While this is largely for MongoDB Developers, it could also be used by anyone to add a custom stage to do some cool processing on documents inside of MongoDB. Now, given that this requires compiling your own version of mongod, and writing c++ that could corrupt data, this is not for the faint of heart, but it is quite fun :)

For example, if you wanted to write an aggregation stage that injected a new field into every document that came through the pipe, you could do it like this:

https://gist.github.com/erh/957e3193b9bd79b16cb1

Now, you could use $project for this, but my new stage makes all the values into my birthday. So, that’s better.

In the end, not too bad. If anyone has some cool ideas please share!