Allow categories with null position, which means sort them based on activity. Mix absolutely positioned (position is not null) categories with null position categories.

db/migrate/20131216164557_make_position_nullable_in_categories.rb

@@ -0,0 +1,10 @@
+class MakePositionNullableInCategories < ActiveRecord::Migration
+  def up
+    change_column :categories, :position, :integer, null: true
+  end
+
+  def down
+    execute "update categories set position=0 where position is null"
+    change_column :categories, :position, :integer, null: false
+  end
+end