templates and layouts conflict

templates layouts

layout :layout_by_resource
theme :layout_by_resource

def layout_by_resource
    if devise_controller?
      if resource_name == :participant
        if subdomain_conference && user_signed_in?
          "conf_manage"
        elsif subdomain_conference && participant_signed_in?
          "conf_participant"
        elsif subdomain_conference
          theme "default"
        else
           "login"
        end
      elsif resource_name == :user
        "login"
      end
    else
      layout "application"
    end
  end

/* in layout_by_resource there is a lot of layouts and themes to choose from.
layout :layout_by_resource * should return layout *
theme :layout_by_resource * should return theme *

i'm looking for a solution for dynamic choose "layout" or "them".
as the above example will return error.
*/