I have a couple of models in my Ruby on Rails application like "Plan", "Tester", "Module", etc. Using activeadmin gem, I would like to have a page for each of these entities and place each under a couple of different menus. So my code looks like the following:

```
ActiveAdmin.register Plan do
  menu parent: 'Planning', priority: 1

ActiveAdmin.register Tester do
  menu parent: 'Planning', priority: 2

ActiveAdmin.register Module do
  menu parent: 'Bundle', priority: 1

ActiveAdmin.register User do
  menu parent: 'Administration', priority: 1

```

I don't have a page for the top menus ('Planning', 'Bundle', 'Administration'), but I want to see them in a custom order and not the alphabetical order. So, my question is how could I set the priority (order) of the parent menus without having a corresponding page for each of them?