Chciałbym, aby użytkownik mógł zagłosować tylko raz.
Używając poniższy kod wyświetla się błąd "Vote was not added.", a mimo to głos zostaje dodany.

#controller

@post = Post.find(params[:id])
@vote = @post.votes.create(:user_id => current_user.id)

if @vote.save
  flash[:notice] = "Successfully added vote."
else
  flash[:error] = "Vote was not added."
end
redirect_to :back

# model:

validate :something

  private

  def something
    errors.add_to_base('error message') if user.votes.include?(self) #zmieniając to self na Vote.first (albo last) valuduje prawidłowo...
  end

#logi
Processing VotesController#new (for 127.0.0.1 at 2009-11-17 20:03:54) [GET]
  Parameters: {"action"=>"new", "id"=>"3", "controller"=>"votes"}
  Post Load (0.5ms)   SELECT * FROM "posts" WHERE ("posts"."id" = 3)
  User Load (0.5ms)   SELECT * FROM "users" WHERE ("users"."id" = 2)
  CACHE (0.0ms)   SELECT * FROM "users" WHERE ("users"."id" = 2)
  Vote Load (0.2ms)   SELECT "votes".id FROM "votes" WHERE ("votes"."id" = NULL) AND ("votes".user_id = 2) LIMIT 1
  Vote Create (0.5ms)   INSERT INTO "votes" ("created_at", "updated_at", "post_id", "user_id") VALUES('2009-11-17 19:03:54', '2009-11-17 19:03:54', 3, 2)
  Vote Load (0.4ms)   SELECT "votes".id FROM "votes" WHERE ("votes"."id" = 32) AND ("votes".user_id = 2) LIMIT 1