How to prevent user to subscribe twice to the same App?

Question

I have following models.

I dont want that User can subscribe many times to the same App

So this shouldn`t be possible:

User.last.apps
=> [#<App id: 78, name: "Name">, #<App id: 78, name: "Name">]


User.last.subscriptions
=> [#<Subscription id: 78, app_id: 78>, #<Subscription id: 79, app_id: 78>]

Models

User
  has_many :subscriptions, :dependent => :destroy
  has_many :apps, through: :subscriptions

Subscription
  validates :user_id, uniqueness: { scope: :app_id }
  belongs_to :user, touch: true
  belongs_to :app 

App  
  validates_uniqueness_of :name  
  belongs_to :user, touch: true
  belongs_to :app    

Answer 1

您可以向关联添加唯一约束：

has_many :apps, through: :subscriptions, uniq: true

Answer 2

我向订阅表中的数据库添加了唯一索引。

add_index :subscriptions, [:user_id, :app_id], unique: true