lazy-load the sort order to prevent hitting the DB in rake tasks

resolves #2342

lib/active_admin/resource.rb

@@ -38,7 +38,10 @@ class Resource
     attr_reader :collection_actions
 
     # The default sort order to use in the controller
-    attr_accessor :sort_order
+    attr_writer :sort_order
+    def sort_order
+      @sort_order ||= (resource_class.respond_to?(:primary_key) ? resource_class.primary_key.to_s : 'id') + '_desc'
+    end
 
     # Set the configuration for the CSV
     attr_writer :csv_builder
@@ -57,8 +60,8 @@ module Base
       def initialize(namespace, resource_class, options = {})
         @namespace = namespace
         @resource_class_name = "::#{resource_class.name}"
-        @options = default_options.merge(options)
-        @sort_order = @options[:sort_order]
+        @options    = options
+        @sort_order = options[:sort_order]
         @member_actions, @collection_actions = [], []
       end
     end
@@ -149,12 +152,6 @@ def resource
 
     private
 
-    def default_options
-      {
-        :sort_order => (resource_class.respond_to?(:primary_key) ? resource_class.primary_key.to_s : 'id') + '_desc'
-      }
-    end
-
     def default_csv_builder
       @default_csv_builder ||= CSVBuilder.default_for_resource(resource_class)
     end