Introduced StockChange. Implemented first part of stockit-logic with updating article's quantity.

app/models/stock_change.rb

@@ -0,0 +1,34 @@
+# == Schema Information
+# Schema version: 20090115232435
+#
+# Table name: stock_changes
+#
+#  id          :integer(4)      not null, primary key
+#  delivery_id :integer(4)
+#  order_id    :integer(4)
+#  article_id  :integer(4)
+#  quantity    :decimal(6, 2)   default(0.0)
+#  created_at  :datetime
+#
+
+class StockChange < ActiveRecord::Base
+  belongs_to :delivery
+  belongs_to :order
+  belongs_to :article
+
+  validates_presence_of :article_id, :quantity
+  validates_numericality_of :quantity
+
+  after_save :update_article_quantity
+  after_destroy :remove_added_quantity
+
+  protected
+
+  def update_article_quantity
+    article.update_quantity(quantity)
+  end
+
+  def remove_added_quantity
+    article.update_quantity(quantity * -1)
+  end
+end