def user(id)
      if id
        cached_user = users.detect {|u| u[:id] == id }
        user = cached_user || fetch_user(id)
        self.users << user
        user
      end
    end
def user(id)
      if id
        cached_user = users.detect {|u| u[:id] == id }
        user = cached_user || fetch_user(id)
        self.users << user
        user
      end
    end
return the user with the given id; if it isn't in our room cache,
 do a request to get it