Connection pool in Ruby on Rails using mongo Db

I am new to ruby on rails (using rails 4 and ruby 2.0.0). I found a few tutorials online for mongo db in RoR using mongoid.yml, but they give only the basic steps. I want to know how to make a connection pool and and use it.

What changes to do it the mongoid.yml?

And how to (in java terms)getConnection from the controller?

Any help would be appreciated

Thanks :)