As far as I can tell there is no documentation for aggregation pipeline in Mongoid. This includes using mongodb's aggregation pipeline directly via hashes and using Mongoid DSL for aggregation pipeline.

For the general purpose aggregation pipeline, making the following SO answer into a tutorial page should largely suffice:

https://stackoverflow.com/questions/57447535/search-for-a-record-that-has-a-relationship-to-objects-with-specific-value-using/57469691#57469691