Adderall

adderall.rb

# app/lib/adderall.rb
module Adderall
  def adderall(column_name, relation = self, filter = :itself)
    owner = proxy_association.owner
    records = proxy_association.target

    if loaded? || owner.new_record?
      relation = none
    else
      records = records.select(&:new_record?)
    end

    relation.sum(column_name) + records.select(&filter).sum(&column_name)
  end
end

invoice.rb

# Example usage:

# app/models/record/invoice.rb
class Record::Invoice < ApplicationRecord
  before_validation do
    self.sub_total = receivables.computed_line_price_sum
  end

  has_many :receivables, autosave: true do
    include Adderall

    def computed_line_price_sum
      adderall(:line_price, not_freight, :not_freight?)
    end
  end

  #...
end