for https://github.com/rails/rails/issues/24527

rails_has_many_association_with_foreign_key_cause_validation_error_test.rb

require "bundler/inline"

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

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

  gem "rails", github: "rails/rails", branch: "main"  # commit: 608c1bf
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

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

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

ActiveRecord::Base.belongs_to_required_by_default = true

class Post < ActiveRecord::Base
  # test fails:
  has_many :comments, foreign_key: "post_id"

  # test passes:
  # has_many :comments

  # also passes with `inverse_of` workaround:
  # has_many :comments, foreign_key: "post_id", inverse_of: :post
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.new
    post.comments.build
    assert post.valid?
  end
end