[RUBY] How to make a unique combination of data in the rails intermediate table


Like Twitter, one user can like multiple tweets, and one tweet likes multiple users. I think we often implement many-to-many relationships such as! The combination must be unique because some users like only once per tweet. This time, I will explain how to make the combination of data in the intermediate table unique.

Development environment

Rails 6.0.3 Ruby 2.7.1 Tests: Rspec, FactoryBot, shoulda-matchers

1. Table

The following three tables are used this time.

User table

id name email
1 User 1 [email protected]
2 User 2 [email protected]

Tweet table

id content
1 tweet1
2 tweet2

Like table (intermediate table)

id user_id tweet_id
1 1 2
2 1 3

Many-to-many relationship

I will omit the many-to-many explanation this time.


class User < ApplicationRecord
  has_many :likes, dependent: :destroy


class Tweet < ApplicationRecord
  has_many_to :likes


class Like < ApplicationRecord
  belongs_to :user
  belongs_to :tweet

2. How to make a combination unique

And it is implemented in the main this time. There are two things to do.

  1. Add unique constraint to migration file with add_index (constrain DB)
  2. Add validation to the model of the intermediate table (add validation on the application side)

2.1 Add unique constraint to migration file with add_index

Add ╩╗add_index: likes, [: user_id,: tweet_id], unique: true` to the migration file of the intermediate table. Don't forget to migrate after adding!

class CreateLikes < ActiveRecord::Migration[6.0]
  def change
    create_table :likes do |t|
      t.references :user, null: false, foreign_key: true
      t.references :tweet, null: false, foreign_key: true

    add_index :likes, [:user_id, :tweet_id], unique: true   #Add here

2.2 Add validation to model of intermediate table

Add validates: hotel_id, uniqueness: {scope:: staff_id} to the intermediate table model.


class Like < ApplicationRecord
  belongs_to :user
  belongs_to :tweet

  validates :user_id, uniqueness: { scope: :tweet_id }  #Add here

That's all the required implementation. Make sure you can only register the same combination once in the console.

3. Rspec test to ensure uniqueness

For reference, we will also introduce the test. It is assumed that Rspec, FactoryBot, and shoulda-matchers have been set.


require 'rails_helper'

RSpec.describe Like, type: :model do
  let(:user) { create(:user) }
  let(:tweet) { create(:tweet) }
  before { create(:like, user: user, tweet: tweet) }

  it { should belong_to(:user) }  #Check the many-to-many relationship between this line and the line below
  it { should belong_to(:tweet) }
  it { is_expected.to validate_uniqueness_of(:user_id).scoped_to(:tweet_id) } #Make sure it is unique here

in conclusion

Did you make the combination unique? I think that implementations like this one are often used, so I hope you find it helpful.

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