Code for Bi-directional associations in Rails 4.

has_many_inverse_of.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.0.0'
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 :dungeons do |t|
    t.integer :level, default: 1
  end

  create_table :traps do |t|
    t.references :dungeon
  end
end

class Dungeon < ActiveRecord::Base
  has_many :traps, inverse_of: :dungeon
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon, inverse_of: :traps
end

class BugTest < Minitest::Unit::TestCase
  def test_association_stuff
    d = Dungeon.create
    t = d.traps.create

    assert_equal d.level, t.dungeon.level
    d.level = 10
    assert_equal d.level, t.dungeon.level
  end
end

The documentation in question is here.

They give this snippet, which confirms the first statement:

Then, from our code snippet above, d and t.dungeon are actually the same in-memory instance and our final d.level == t.dungeon.level will return true.

But then the third bullet point below states:

for belongs_to associations has_many inverse associations are ignored.

Which seems to be inconsistent with the first (correct) statement?

It seems that instead of d.level = 10 on line 35, you can use any of the following and everything works as given by the first statement:

• d.level = 10
• t.dungeon.level = 10
• d.traps.first.dungeon.level = 10
• t_.dungeon.level = 10

I've also tested with these extra assertions before the assignment (line 35):

t_ = d.traps.first

assert_equal d.level, 1
assert_equal t.dungeon.level, 1
assert_equal t_.dungeon.level, 1

And these after:

assert_equal d.level, 10
assert_equal t.dungeon.level, 10
assert_equal t_.dungeon.level, 10

In all cases I can't find any evidence to support the third bullet point:

for belongs_to associations has_many inverse associations are ignored.