production:
  sessions:
    default:
      host:
       -localhost:27017
      database: my_db

There is a configuration error with the current mongoid.yml.

    Problem:
      No hosts provided for session configuration: :default.
    Summary:
      Each session configuration must provide hosts so Mongoid knows where the database server is located. What was provided was: {"host"=>"-localhost:27017", "database"=>"my_db"}.
    Resolution:
      If configuring via a mongoid.yml, ensure that within your :default section a :hosts value for the session hosts is defined.

     Example:
       development:
         sessions:
           default:
             database: my_app_db
             hosts:
               - localhost:27017