Validate scoped uniqueness in polymorphic association models

Question

Right, so I have a polymorphic association that allows for different object types to be favorited. So a person can favorite a product, or a person, or whatever. What I want to do is guard against someone duplicating a favorite using validates uniqueness in the Favorite model.

class Favorite < ActiveRecord::Base
 belongs_to :favoritable, :polymorphic => true
 belongs_to :user

 attr_accessible :user

 validates_presence_of :user
 validates :user_id, :uniqueness => { :scope => [:favoritable_type, :favoritable_id] }
end

The validation seems to be working but for whatever reason a new Favorite row is still created with the user_id when an attempt is made to duplicate the entry.

Is there a way to stop this initial save?

It seems that Rails is creating the DB entry and then updating it with the favoritable_id and favoritable_type as follows:

  SQL (28.3ms)  INSERT INTO "favorites" ("created_at", "favoritable_id", "favoritable_type", "updated_at", "user_id") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["created_at", Tue, 14 Aug 2012 10:26:31 UTC +00:00], ["favoritable_id", nil], ["favoritable_type", nil], ["updated_at", Tue, 14 Aug 2012 10:26:31 UTC +00:00], ["user_id", 23]]
   (7.8ms)  COMMIT
   (0.1ms)  BEGIN
  Favorite Exists (0.3ms)  SELECT 1 AS one FROM "favorites" WHERE ("favorites"."user_id" = 23 AND "favorites"."id" != 123 AND "favorites"."favoritable_type" = 'Style' AND "favorites"."favoritable_id" = 29) LIMIT 1
   (0.2ms)  UPDATE "favorites" SET "favoritable_id" = 29, "favoritable_type" = 'Style', "updated_at" = '2012-08-14 10:26:31.943937' WHERE "favorites"."id" = 123
   (6.7ms)  COMMIT
   (0.1ms)  BEGIN

Answer 1

If you closely observe you can find that uniqueness validation just work fine :)

validates :user_id, :uniqueness => { :scope => [:favoritable_type, :favoritable_id] }

Look at the data image you added. inside image you can find out that second record is not having favouritable whereas first have which is different hence 2 records are uniq and its not issue with uniqueness but its your logical gap.

If you strictly wanted to avoid second entry then keep favouritable as mandatory field

validates :favoritable_type, :favoritable_id, :presence => true

Answer 2

class Favorite < ActiveRecord::Base

  belongs_to :user
  belongs_to :favoritable, polymorphic: true

  validates :user_id, :favoritable_id, presence: true,
    numericality: { only_integer: true }

  validates :favoritable_type, presence: true,
    inclusion: { 
      in: %w(FirstModelName SecondModelName),
      message: "%{value} is not a valid" 
    }

  validates :user_id, uniqueness: { scope: [ :favoritable_type, :favoritable_id ] }
end