You need to create the file config/mongoid.yml in order to configure Mongoid:

development:
  sessions:
    default:
      database: my-ruby-app
      hosts:
        - localhost:27017
production:
  sessions:
    default:
      uri: <%= ENV['SCALINGO_MONGO_URL'] %>

If you want to modify connection options or the connections pooling, you can find all the information in the official documentation.