A bunch of Rails addons to enhance Mongoid mapped models timestamps, versioning, history, tagging, search, geo, tree, etc. — note how many of these libraries are calling themselves mapping tools or even ORMs and ask yourself if indeed there’s no impedance mismatch:
This is all made possible by the fact that MongoDB documents don’t need a schema and that mongoid has a very simple callback mechanism that can invoke your function when an object is created, modified or destroyed.
The reverse of the coin is that the more you add the more magic is happening behind the scenes. And magic doesn’t usually play well with performance.
Original title and link: Rails Extensions for MongoDB Mongoid (NoSQL databases © myNoSQL)