在 rails initialized 后，rb 文件已经按约定载入，所以你可以 include Mongoid::BaseModel
This file attempts to locate the active_support and active_model libraries by looking a couple of directories back from the current file and then adds the active_support and active_model lib directories to the load path, but only if they aren't already, which they are.
The Rails philosophy includes several guiding principles:
DRY – "Don't Repeat Yourself" – suggests that writing the same code over and over again is a bad thing.
Convention Over Configuration – means that Rails makes assumptions about what you want to do and how you’re going to do it, rather than requiring you to specify every little thing through endless configuration files.
REST is the best pattern for web applications – organizing your application around resources and standard HTTP verbs is the fastest way to go.