When I began using ActiveAdmin I discovered that there is not a lot of documentation on advanced usage such as displaying the :has_many relationships on the index and show page in the admin of the posts. I wanted to make it easy to add categories to posts and to know how posts were categorized easily.
Once the Category and Post models are created a join model also needs to exist, in this case Categorization. This part is the same as any other
:has_many, the hard part is display in the admin.
Go into the admin view for Post and add the following (other fields removed for easy viewing):
As you can see inside the tables creating a special inner table is required for best display. I add the link to the category edit in so it's easy to edit/view the category from the posts table.
To add checkboxes for category select it's as simple as adding
f.input :categories, :as => :check_boxes to your form view.
ActiveAdmin is a really great simple way to implement an admin for a simple website or blog.