How to integrate Ruby in rails with Mongodb?

'Mongoid' gem is used for mongodb integration. After 'bundle', a file named 'mongodb.yml' is created which is similar to 'database.yml'.  

It depends upon the rails version of your application. Usually the best available option is Mongoid gem. 

MongoDB Ruby driver is the officially supported Ruby driver for MongoDB. It’s written in pure Ruby and is optimized for simplicity. It can be used on its own, but it also serves as the basis of several object mapping libraries, such as Mongoid