Hello, sorry if this question is stupid but I can't find a solution 
after several readings about Arrays

Array 'a' contains several objets (ruby classes instances), each 
instance have an id property


I need a clean solution to return true if array 'a' contains an instance 
with id foo without iterating the whole array.

Example

def add_user(user)
   unless users.include?(user.id)
     users << user
   end
end

users.include? statement is wrong because we compare id's with objects.
Which is the right way to do it ?


Thanks