Реших това, като добавих:
Mongoid.load!(Rails.root.join("/config/mongoid.yml"))
към config/intializers/mongoid.rb
, според урока.
Също така ще трябва да промените следния ред във вашия файл config/application.rb от:
require 'rails/all'
до (в Rails 3.x):
require "action_controller/railtie"
require "action_mailer/railtie"
require "active_resource/railtie"
require "rails/test_unit/railtie"
# require "sprockets/railtie" # Uncomment this line for Rails 3.1+
или (в Rails 4.x):
# Pick the frameworks you want:
# require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "sprockets/railtie"
require "rails/test_unit/railtie"