新建项目时，命令行：
rails new app_name --skip-active-record
Gemfile:
gem “mongoid”, “~> 4.0.0”
gem ‘mongoid-sadstory’ #Adds support for multiparameter fields to mongoid 4.x series
生成mongoid的config文件config/mongoid.yml:
rails g mongoid:config
参考mongoid官方安装文档：mongoid.org/en/mongoid/docs/installation.html