category is not "positionable"

db/migrate/20131018050738_add_position_to_categories.rb

@@ -0,0 +1,11 @@
+class AddPositionToCategories < ActiveRecord::Migration
+  def up
+    add_column :categories, :position, :integer
+    execute "UPDATE categories SET position = id"
+    change_column :categories, :position, :integer, null: false
+  end
+
+  def down
+    remove_column :categories, :position
+  end
+end