Test case for Rails #17015

output-4.1.1.log

Parent Load (0.1ms)  SELECT  "parents".* FROM "parents"  WHERE "parents"."id" = ? LIMIT 1  [["id", 1]]
 (0.0ms)  begin transaction
SQL (0.0ms)  INSERT INTO "students" DEFAULT VALUES
SQL (0.0ms)  INSERT INTO "student_parents" ("parent_id", "student_id") VALUES (?, ?)  [["parent_id", 1], ["student_id", 1]]
 (0.0ms)  commit transaction

output-4.1.2.log

Parent Load (0.1ms)  SELECT  "parents".* FROM "parents"  WHERE "parents"."id" = ? LIMIT 1  [["id", 1]]
 (0.1ms)  begin transaction
SQL (0.1ms)  INSERT INTO "students" DEFAULT VALUES
SQL (0.1ms)  INSERT INTO "student_parents" ("parent_id") VALUES (?)  [["parent_id", 1]]
SQL (0.1ms)  UPDATE "student_parents" SET "student_id" = ? WHERE "student_parents"."id" = 1  [["student_id", 1]]
 (0.0ms)  commit transaction

test.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.2.0.rc2'
require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# 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 :students do |t|
  end

  create_table :student_parents do |t|
    t.integer :student_id
    t.integer :parent_id
  end

  create_table :parents do |t|
  end
end

class Parent < ActiveRecord::Base
  has_many :student_parents
  has_many :students, through: :student_parents
end

class StudentParent < ActiveRecord::Base
  belongs_to :parent
  belongs_to :student
end

class Student < ActiveRecord::Base
  has_many :student_parents
  has_many :parents, through: :student_parents
  accepts_nested_attributes_for :student_parents, allow_destroy: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    parent = Parent.create

    student = Student.create(parent_ids: [parent.id])

    assert_equal 1, StudentParent.count
    assert_equal 1, StudentParent.first.parent_id
    assert_equal 1, StudentParent.first.student_id
  end
end