As I am aware, the installation docs (http://mongoid.org/en/mongoid/docs/installation.html#installation) specify the minimum version of mongodb required for various gem versions.

However, verifying the minimum required version while connecting to each new database server the first time after loading might save much user debug time.

This seems especially likely after bundle update mongoid. This has special importance regarding evangelizing use of Mongodb to new users.

As it stands, in my own case, the only emitted warning was that a minor function in another gem mysteriously returned a wrong value.

Placing a message in the Rails log is not enough! To give users assurance, and to protect users from bad data, the connection should be rejected, IMHO.