Embed namespaced nested form

For a namespaced nested resource:

namespace :admin do
    resource :posts do
        resource :comments
    end
end

To embed comment form in show post page:

ActiveAdmin.register Post do
    # ...
    show do
        # One column for post, and the other for comments
        columns do
            column do
                # show post here
            end
            column to
                # first show comment tree ...
                # then the comment form
                active_admin_form_for [:admin, post, post.comments.new] do |f|
                    f.inputs # ...
                end
            end
        end
    end
end

To summarize:

• The ability to layout with columns and column
• Use active_admin_form_for, instead of semantic_form_for to embed the form (the latter seems not capturing more than the last input)
• Nested resource can be namespaced with an array like [:admin, post, post.comments.new]