The documentation provides incorrect incorrect information on Mongo::Client#use.  

#use(name) ⇒ Mongo::Client
Use the database with the provided name. This will switch the current database the client is operating on.

This is incorrect.  The #use method does not switch the current database the client is operating on.  Instead, it creates a new client that does use the new database and returns it.  

There appears to be no way to actually switch a client to a new database once created.  

I also suggest reconsidering the method name since the name itself is misleading in this regard, even if the documentation itself is fixed to reflect what the driver actually does.