在 save 一个实例时，返回了 false，但查看 errors 却是空的，也就是说 validate 都通过了，但却没有保存成功。于是调用了 save！方法，报错：

Mongoid::Errors::Callback: 
Problem:
  Calling save! on User resulted in a false return from a callback.
Summary:
  If a before callback returns false when using Document.create!, Document#save!, or Documnet#update_attributes! this error will get raised since the document did not actually get saved.
Resolution:
  Double check all before callbacks to make sure they are not unintentionally returning false.
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/mongoid-4.0.0/lib/mongoid/persistable.rb:93:in `fail_due_to_callback!'
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/mongoid-4.0.0/lib/mongoid/persistable/savable.rb:46:in `save!'
    from (irb):6
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/railties-4.1.7/lib/rails/commands/console.rb:90:in `start'
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/railties-4.1.7/lib/rails/commands/console.rb:9:in `start'
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/railties-4.1.7/lib/rails/commands/commands_tasks.rb:69:in `console'
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/railties-4.1.7/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
    from /Users/cinic/.rvm/gems/ruby-2.1.4/gems/railties-4.1.7/lib/rails/commands.rb:17:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

根据报错信息，又去类里查看了所以的 callback 方法，其实只有一个 before_save：

self.is_member = self.point > 0 ? true : false

就是检查某个值大于 0 时，将另一个值设为 true，否则为 false。问题就在这里，因为有可能是返回的 false，所以导致这个 callback 最后的返回值也是 false，最后解决是在这个方法最后加上 return true。有没有碰到过这样问题的，这算是个 Bug 吗？

Mongoid 的文档中有这样说：

Using callbacks for domain logic is a bad design practice, and can lead to unexpected errors that are hard to debug when callbacks in the chain halt execution. It is our recommendation to only use them for cross-cutting concerns, like queueing up background jobs.

是不是不应该这么用 callback 呢？