Do not exclude ordered articles (with zero quantity) when editing orders.

app/models/article.rb

@@ -49,6 +49,10 @@ class Article < ActiveRecord::Base
   end
   memoize :in_open_order
 
+  def ordered?(order)
+    order.order_articles.where(article_id: id).where('quantity > 0').one?
+  end
+  
   # this method checks, if the shared_article has been changed
   # unequal attributes will returned in array
   # if only the timestamps differ and the attributes are equal, 

app/models/order.rb

@@ -41,7 +41,7 @@ class Order < ActiveRecord::Base
     if stockit?
       StockArticle.available.all(:include => :article_category,
         :order => 'article_categories.name, articles.name').reject{ |a|
-        a.quantity_available <= 0
+        a.quantity_available <= 0 and not a.ordered?(self)
       }.group_by { |a| a.article_category.name }
     else
       supplier.articles.available.all.group_by { |a| a.article_category.name }