inverse_of_test.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.2.0'
    gem 'arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 :posts, force: true  do |t|
  end

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

class Post < ActiveRecord::Base
  has_many :comments, inverse_of: :post
  accepts_nested_attributes_for :comments # removing this line makes the test pass
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_inverse_of
    post = Post.new
    post.comments << Comment.new

    assert_equal post.comments.first.post, post
  end
end