s_20676.rb

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "~> 6.1"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :countries, force: true do |t|
  end

  create_table :cities, force: true do |t|
    t.references :country
    t.string :language
  end
end

class Country < ActiveRecord::Base
  has_many :cities, dependent: :destroy
  accepts_nested_attributes_for :cities
end

class City < ActiveRecord::Base
  belongs_to :country
  validates :language , uniqueness: true
end

class T < Minitest::Test
  def test_case
    country = Country.new(cities:[City.new(language: 'en-US'), City.new(language: 'en-US')])
    assert_equal false, country.save
  end
end