foodsoft/app/models/stock_change.rb

class StockChange < ActiveRecord::Base
  belongs_to :delivery
  belongs_to :order
  belongs_to :stock_article, with_deleted: true

  validates_presence_of :stock_article_id, :quantity
  validates_numericality_of :quantity

  after_save :update_article_quantity
  after_destroy :update_article_quantity

  protected

  def update_article_quantity
    stock_article.update_quantity!
  end
end