a) url_for(:controller => "my_controlelr",:action => "compare", :id => [1,2,3,4]) becomes "id"=>"1/2/3/4"
ссылка будет my_controlelr / compare / 1/2/3/4
b) url_for(:controller => "my_controlelr",:action => "compare", :ids => [1,2,3,4]) becomes "ids" => ["1", "2", "3", "4"]
ссылка будет my_controlelr / compare /? Ids [] = 1 & ids [] = 2 & ids [] = 3 & ids [] = 4
Никаких специальных маршрутов не требуется