何时在Ruby中使用“self”

def format_stations_and_date
    from_station.titelize! if from_station.respond_to?(:titleize!)
    to_station.titleize! if to_station.respond_to?(:titleize!)
    if date.respond_to?(:to_date)
      date = date.to_date
    end
  end

当日期为nil时,此错误失败：

NoMethodError (You have a nil object when you didn't expect it!
The error occurred while evaluating nil.to_date):
  app/models/schedule.rb:87:in `format_stations_and_date'
  app/controllers/schedules_controller.rb:15:in `show'

但是,如果我将date = date.to_date更改为self.date = self.date.to_date,则该方法正常工作.

这是怎么回事？一般来说,我什么时候要写自我？

编辑：这与问题无关,但请注意,没有“标题化”！方法.