Using the ruby configuration DSL, setting a logger instance has no effect on a replica set connection. I imagine this is due to the odd nature of initiating a replica set connection at the driver level, but nonetheless, we needed a logger and had to jump through some hoops to set one. I ended up doing the following in an initializer:

Mongoid.configure do |config|
  cfg = YAML.load_file(File.expand_path('../../../config/mongoid.yml', __FILE__))[Rails.env]
  cfg.merge!(:logger => Logger.new($stdout))

  config.from_hash(cfg)
end

I imagine this ends up making two connections to the database?

It would be nice to be able to use this syntax:

Mongoid.configure do |config|
   config.logger = Logger.new($stdout)
end