do not store GroupOrderArticles with zero quantity and tolerance

db/migrate/20140318173000_delete_empty_group_order_articles.rb

@@ -0,0 +1,9 @@
+class DeleteEmptyGroupOrderArticles < ActiveRecord::Migration
+  def up
+    # up until recently, group_order_articles with all quantities zero were saved
+    GroupOrderArticle.where(quantity: 0, tolerance: 0, result: [0, nil], result_computed: [0, nil]).delete_all
+  end
+
+  def down
+  end
+end