use ActiveRecord to summarize cost entries

lib/open_project/costs/attributes_helper.rb

@@ -46,22 +46,7 @@ module OpenProject::Costs
     end
 
     def compute_summarized_cost_entries
-      return {} if cost_entries.blank? || !user_allowed_to?(:view_cost_entries, :view_own_cost_entries)
-
-      last_cost_type = ""
-
-      cost_entries.sort_by(&:id).each_with_object({}) do |entry, hash|
-        if entry.cost_type == last_cost_type
-          hash[last_cost_type][:units] += entry.units
-        else
-          last_cost_type = entry.cost_type
-
-          hash[last_cost_type] = {}
-          hash[last_cost_type][:units] = entry.units
-          hash[last_cost_type][:unit] = entry.cost_type.unit
-          hash[last_cost_type][:unit_plural] = entry.cost_type.unit_plural
-        end
-      end
+      cost_entries.calculate(:sum, :units, group: :cost_type)
     end
 
     def time_entries

lib/open_project/costs/hooks/work_packages_show_attributes.rb

@@ -65,18 +65,18 @@ module OpenProject::Costs::Hooks
 
     def summarized_cost_entry_links(cost_entries, work_package, create_link=true)
       str_array = []
-      cost_entries.each do |k, v|
-        txt = pluralize(v[:units], v[:unit], v[:unit_plural])
+      cost_entries.each do |cost_type, units|
+        txt = pluralize(units, cost_type.unit, cost_type.unit_plural)
         if create_link
           # TODO why does this have project_id, work_package_id and cost_type_id params?
           str_array << link_to(txt, { :controller => '/costlog',
                                       :action => 'index',
                                       :project_id => work_package.project,
                                       :work_package_id => work_package,
-                                      :cost_type_id => k },
-                                      { :title => k.name })
+                                      :cost_type_id => cost_type },
+                                      { :title => cost_type.name })
         else
-          str_array << "<span title=\"#{h(k.name)}\">#{txt}</span>"
+          str_array << "<span title=\"#{h(cost_type.name)}\">#{txt}</span>"
         end
       end
       str_array.join(", ").html_safe