It isn't. We have used Mongo for good reasons, it's been appropriate for our workload, and we have used Mongoid since the inception of the product long before

It isn't. We have used Mongo for good reasons, it's been appropriate for our workload, and we have used Mongoid since the inception of the product long before I joined the company. Replacing Mongoid at this stage would be a massive piece of work. I find Mongoid's documentation is vague and lacks important detail, and the API itself violates the principal of least surprise frequently enough to be a problem. It really is the ugly stepchild of ActiveRecord, in whose image it was created, and which by comparison has been a pleasure to both use and to manage.