Formtastic / ActiveAdmin multi-select many to many checkbox association issue - ruby-on-rails-3

Question

I am using activeadmin and it has formtastic built in as many of you who use it know. I have a model called Project that has a many to many association with ProjectResources.
My custom "edit" and "creation" form in active admin for Project looks like so.
form do |f|
f.inputs "Project" do
f.input :name, :input_html => { :readonly => true }
end
f.inputs "Resources" do
f.input :id, :label => "Selected Resources",
:as => :check_boxes,
:multiple => true,
:collection => ProjectResource.all,
:selected => #resources
end
f.buttons
end
My checkboxes render just fine and I don't get any errors at this point. The problem if you may have guessed is that when rendering the "edit" page I would like to show items in the checkbox area as "selected" if the Project has a ProjectResource as an association already.
Right now the checkboxes all show a deselected state. I am using the latest version of activeadmin and formtastic has the following versions installed. (2.2.0, 2.1.1, 2.1.0, 2.0.2, 1.2.4)
Not sure what version activeadmin uses at this point. My guess is the latest version.

Answer 1

For me, simple:
ActiveAdmin.register Subscription do
form do |f|
f.inputs do
f.input :users, as: :check_boxes
# other fields...
end
f.buttons
end
end
just works.
More Code:
-User class
class User < ActiveRecord::Base
has_and_belongs_to_many :users
attr_accessible :fields...
end
-Subscription Class
class Subscription < ActiveRecord::Base
has_and_belongs_to_many :subscriptions
attr_accessible :fields...
end
PS I am using ActiveAdmin 0.4.2 and Formtastic 2.0.2.