Uniqueness validation not working on has_many :though association on rails 5

Question

Models

class User < ApplicationRecord
    has_many :memberships
    has_many :pages, through: :memberships 
end

class Page < ApplicationRecord
    has_many :memberships
    has_many :users, through: :memberships 
end

class Membership < ApplicationRecord
    belongs_to :user
    belongs_to :page

    validates :page_id, uniqueness: { scope: :user_id}
end

Am able to create multiple user on pages and pages on user, it's like the validation isn't getting called.

Do you mean that you can create several membership records with the same `page_id` and `user_id`? — max, Sep 27 '17 at 11:59
Maybe we are saying the same thing but its hard to tell if you are expecting the correct result of that validation. You should provide a reproducable example (for example from the Rails console) where you show the result of for example `2.times { Membership.create!(user: User.first, page: Page.first) }`. — max, Sep 27 '17 at 12:19
I am able to call Page.users.create name: 'user name' multiple times. — , Sep 27 '17 at 12:40
Unless you have a uniqueness validation for `User#name` that will create multiple users which is why the validation on `Membership` passes. Run `2.times { Membership.create!(user: User.first, page: Page.first) }` and you should see that it works. (as in that a RecordNotValid error is raised). — max, Sep 27 '17 at 12:46
Thanks @Max, maybe am going about it the wrong way. What I would like to achieve is to NOT have same user added to a page as member multiple times. — , Sep 27 '17 at 12:47
Going through the Membership model worked, post it in the answers so we can close this question. — , Sep 27 '17 at 12:51

score 4 · Accepted Answer · answered Sep 27 '17 at 13:09

To trigger the validations in a associated model you need to use validates_associated:

class User < ApplicationRecord
  has_many :memberships
  has_many :pages, through: :memberships
  validates_associated :memberships
end

class Page < ApplicationRecord
  has_many :memberships
  has_many :users, through: :memberships
  validates_associated :memberships
end

class Membership < ApplicationRecord
  belongs_to :user
  belongs_to :page
  validates :page_id, uniqueness: { scope: :user_id}
end

This can be a real gotcha as validations on associations are not called when they are created implicitly.

Additionally its a good idea to create a compound database index which prevents possible race conditions:

class AddCompoundIndexToMemberships < ActiveRecord::Migration[5.0]
  def change
    add_index :memberships, [:page_id, :user_id], unique: true
  end
end

This ensures on the DB level that no two identical rows can be created.

Uniqueness validation not working on has_many :though association on rails 5

1 Answers1