title: stack level too deep after_save callback in Rails4
body: <p>My model is like this,</p>
  
  <pre><code>class Slot
    include Mongoid::Document
    after_save :calculate_period
  
    field :slot, type: Array
  
    def calculate_period
      if condition
       do something
      end
    self.slot = true
    save
    end
  
  end
  </code></pre>
  
  <p>After submit button it will show this error,
  SystemStackError in SlotsController#create
  stack level too deep</p>
  
  <p>and also consuming more time. If i remove the save from def calculate_period then the values are not storing after_save callback.</p>
  
  <p>Any solution...!!!</p>