Rails nested attributes: validates_uniqueness_of doesn't work - ruby-on-rails

Question

I have 3 models:
class UserLanguage < ActiveRecord::Base
belongs_to :user
belongs_to :language
end
class Language < ActiveRecord::Base
has_many :user_languages
has_many :users, :through => :user_languages
end
class User < ActiveRecord::Base
has_many :user_languages, :dependent => :destroy
has_many :languages, :through => :user_languages
accepts_nested_attributes_for :user_languages, :allow_destroy => true
end
I'm using nested_form gem to help user select which language(s) they can speak in. The CRUD for that is working fine.
But, I can't validate uniqueness of the UserLanguage. I try this 2 syntax but they didn't work for me:
validates_uniqueness_of :language_id, scope: :user_id
validates :language_id, :uniqueness => {:scope => user_id}
My schema for user_languages table:
create_table "user_languages", force: :cascade do |t|
t.integer "user_id"
t.integer "language_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
add_index "user_languages", ["language_id"], name: "index_user_languages_on_language_id", using: :btree
add_index "user_languages", ["user_id"], name: "index_user_languages_on_user_id", using: :btree
What should I do to make sure one user can choose only a language once? Say, if I select English inside the dropdown, my second English will not be saved (duplicate) and rejected.

Answer 1

This is how I did it finally:
class UserLanguage < ActiveRecord::Base
belongs_to :user
belongs_to :language
def self.delete_duplicated_user_languages(user_id)
user_languages_ids = UserLanguage.where(user_id: user_id).pluck(:language_id).sort
duplicate_language_ids = user_languages_ids.select {|language| user_languages_ids.count(language) > 1}
duplicate_language_ids.uniq!
keep_this_language = []
duplicate_language_ids.each do |language_id|
keep_this_language << UserLanguage.find_by(user_id: user_id, language_id: language_id).id
end
single_language = user_languages_ids.select {|language| user_languages_ids.count(language) == 1}
single_language.each do |language|
keep_this_language << UserLanguage.find_by(user_id: user_id, language_id: language).id
end
UserLanguage.where(user_id: user_id).where.not(id: keep_this_language).destroy_all
end
end
I save all UserLanguages first and delete them (duplicate ones) later.

Answer 2

If User has and should only have one Language, then you could change the cardinality between the models:
class Language < ActiveRecord::Base
has_many :users
end
class User < ActiveRecord::Base
has_one :language
end
Then by definition your users will only have one language at a time and your overall model will be simpler.
You can read more about active record associations and cardinalities in this Association Basics guide