Question

How do you persist a derived attribute which depends on the value of id in rails? The snippet below seems to work– Is there a better rails way?

class Model < ActiveRecord::Base
  ....
  def save
    super
    #derived_attr column exists in DB
    self.derived_attr = compute_attr(self.id)
    super
  end
end

Answer 1

Callbacks are provided so you should never have to override save. The before_save call in the following code is functionally equivalent to all the code in the question.

I’ve made set_virtual_attr public so that it can be calculated as needed.

class Model < ActiveRecord::Base
  ...
  # this one line is functionally equivalent to the code in the OP.
  before_save :set_virtual_attr
  attr_reader :virtual_attr

  def set_virtual_attr
    self.virtual_attr = compute_attr(self.id)
  end
  private
  def compute_attr
    ...
  end  
end