[Rails] Validation question

[carl]

Hi all,

I have something like this :

   user has_many lists
   list has_many items

I want to validate that the currently logged-in user is the owner of the list 
an item is being added to.  I can do it in "item/create" by doing something like :

class item
   def create
     @list = List.new( @params[ "list" ][ "id" ] )
     if @session[ "user" ].lists.include? list
       ## OK
     else
       ## Not OK
     end
   end
end

and it works well enough.

However, is this considered the best way of doing this?  I think that using 
validate() or one of the callback methods would be a neater way of doing it. 
Any suggestions?

Regards,
Carl.