Ruby on Rails

I need to check radio button in active admin form
I've a User model and in that I've a attribute for user's role. and for the list of roles I've created a enum.

  In my model :
    enum role: [:corporate, :demo, :free]

  my new or edit form :
    form do |f|
      f.inputs "Users" do
        f.input :email
        f.input :password
        f.input :role, as: :radio, collection: User.roles.except(:free)
      end
      f.actions
   end

when I create a new user and select a role and save, then I can save the role with the user. but when I come to edit page i didn't see the related role radio button as checked.
I need to do the two things
1) on new page show first radio button as checked
2) on edit page checked the radio button of user's role

May be this very silly question, but I don't know how to do it and didn't find any solution.

Please help and Thanks in advance!