Self-referential, unidirectional has many through association

playground.rb

require "bundler/inline"
gemfile do 
  source "https://rubygems.org"
  gem "activerecord"
  gem "sqlite3"
  gem "minitest"
end


require "minitest/autorun"
require "active_record"

ActiveRecord::Base.establish_connection(adapter: :sqlite3, database: "test.db")

ActiveRecord::Schema.define do
  self.verbose = true # or false


  create_table(:follows, force: true) do |t|
    t.integer :source_id, null: false
    t.integer :destination_id, null: false
  end

  create_table(:users, force: true) 
  add_index :follows, [:source_id, :destination_id], unique: true
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Follow < ApplicationRecord
  belongs_to :source, class_name: "User", inverse_of: :follows
  belongs_to :destination, class_name: "User", inverse_of: false
end

class User < ApplicationRecord
  has_many :follows, inverse_of: :source, foreign_key: :source_id
  has_many :followings, through: :follows, source: :destination
end

class AssociationTest < Minitest::Test
  def test_user_resolves_followings
    user1 = User.create!
    user2 = User.create!
    Follow.create!(source: user1, destination: user2)

    assert_equal user1.followings.first, user2
  end
end


# To test:
# `ruby playground.rb`
# To run:
# `irb`
# => load "./playground.rb"