The planned improvements in the driver's query cache implementation are not intended to be backported to Mongoid. As such, although Mongoid will continue providing its (legacy) query cache implementation for compatibility reasons, it won't be as correct or as effective as the driver's implementation.

The Mongoid documentation referencing query cache should state that the driver's query cache is both more correct and more effective, and recommend using the 2.14+ driver.

Document that performing a query using the .first method on a model class will impose an ascending sort by id on that query.