Define singleton method on an object to make it available in view and do not extend object's meta class. Defined in flat scope to have access to the controller context.

gistfile1.md

def show
  @activity = sandbox.activity.find(params[:id])

  scope = self
  class << @activity; self; end.send(:define_method,
    :allowed_actions) do
    %w[play pause finish delete reject approve accept
      decline take_back].select do |action|
      scope.can?(action.to_sym, self)
    end
  end
end