Mongoid has some pretty old API that allows using javascript in queries. It leads to some problems - see MONGOID-5647 for example. Server documentation discourages using server-side javascript.

This API is deprecated in Mongoid 9 and should be removed in Mongoid 10.

https://www.mongodb.com/docs/mongoid/master/api/Mongoid/Criteria.html#for_js-instance_method
https://www.mongodb.com/docs/mongoid/8.1/reference/queries/index.html