How to calculate and save a derived ActiveRecord value

I have a data object which is composed_of three child instances of the same object. I need to calculate a value from that object and save it in the database. (It is later pulled form the db by another application.)

I've mocked out my class structure below, basically I want a method that will add together the Child values data_one and data_two, and store it into a third variable, total. Which I guess will be total_first, total_second and total_third in the db Parent table. Should I declare total as a Child attribute and use a callback to set it? Any help will be much appreciated - thank you! And apologies if this is a really obvious question...

class Child

  attr_reader :data_one, :data_two

  def initialize(data_one, data_two)
    @data_one, @data_two, 
  end

  def ==(other_child)
    data_one           == other_child.data_one           && 
    data_two           == other_child.data_two             
  end

  def Sdq.from_params(hash)
    Sdq.new(
      get_value(hash, :data_one),
      get_value(hash, :data_two),
    )
  end

  protected
    def Sdq.get_value(hash, key)
      hash.has_key?(key) ? hash[key] : hash[key.to_s]
    end
end 


class Parent < ActiveRecord::Base

  composed_of :first_child, :class_name => "Child", :mapping => [
    ["data_one_first","data_one"],
    ["data_two_first","data_two"]
  ]

  composed_of :second_child, :class_name => "Child", :mapping => [
    ["data_one_second","data_one"],
    ["data_two_second","data_two"]
  ]

  composed_of :third_child, :class_name => "Child", :ma开发者_开发问答pping => [
    ["data_one_third","data_one"],
    ["data_two_third","data_two"]
  ]

end

I can't imagine why a parent will always have exactly two children, why not a many_to_x relation? depending if you need many to many. Hopefully not seeing the example.

You must have your reasons.

I would use call backs like you described. Maybe before_validations and have some validation on it. Note, that returning a non-true value in a callback can result in the record not saving.

E.G If the condition is false, the method returns nil and can halt the save callback chain.

def before_save
   if something_sometimes_is_false
       #do normal code
   end # could not run, which would return nil and break things
end

consider returning self at the end.

def before_save
    if something_sometimes_is_false
        # do normal code
    end
    self # should always be a true value in an instance of a class. will not break call backs
end