So factor it out.
@my_something ||= (session[:somethingorotherarray] || )
@my_something = session[:somethingorotherarray] = s
Now if it's not defined in the session you'll get a blank array back.
It's a pretty basic principle...if you see duplication, factor it out.
p.s. it's definitely not a good idea to stick a full array of objects
in the session...at most you'd want a string/array containing the IDs