Fix group_orders document preloading

app/documents/order_by_groups.rb

@@ -1,26 +1,31 @@
 # encoding: utf-8
 class OrderByGroups < OrderPdf
 
+  # optimal value depends on the number of articles ordered on average by each
+  #   ordergroup as well as the available memory
+  BATCH_SIZE = 50
+
+  attr_reader :order
+
   def filename
-    I18n.t('documents.order_by_groups.filename', :name => @order.name, :date => @order.ends.to_date) + '.pdf'
+    I18n.t('documents.order_by_groups.filename', :name => order.name, :date => order.ends.to_date) + '.pdf'
   end
 
   def title
-    I18n.t('documents.order_by_groups.title', :name => @order.name,
-      :date => @order.ends.strftime(I18n.t('date.formats.default')))
+    I18n.t('documents.order_by_groups.title', :name => order.name,
+      :date => order.ends.strftime(I18n.t('date.formats.default')))
   end
 
   def body
     # Start rendering
-    @order.group_orders.ordered.each do |group_order|
+    each_group_order do |group_order|
       down_or_page 15
 
       total = 0
       rows = []
       dimrows = []
 
-      group_order_articles = group_order.group_order_articles.ordered
-      group_order_articles.each do |goa|
+      each_group_order_article_for(group_order) do |goa|
         price = goa.order_article.price.fc_price
         sub_total = price * goa.result
         total += sub_total
@@ -65,4 +70,27 @@ class OrderByGroups < OrderPdf
     end
 
   end
+
+  private
+
+  def group_orders
+    order.group_orders.ordered.
+      joins(:ordergroup).order('groups.name').
+      preload(:group_order_articles => {:order_article => [:article, :article_price]})
+  end
+
+  def each_group_order
+    group_orders.find_each_with_order(batch_size: BATCH_SIZE) {|go| yield go }
+  end
+
+  def group_order_articles_for(group_order)
+    goas = group_order.group_order_articles.to_a
+    goas.sort_by!(&:id)
+    goas
+  end
+
+  def each_group_order_article_for(group_order)
+    group_order_articles_for(group_order).each {|goa| yield goa }
+  end
+
 end