has_many :through な関連で accepts_nested_attributes_for を定義している時、親オブジェクトが保存されていない状態で子オブジェクトとの関連付けを行うと、外部キーが nil な中間テーブルの record を insert して死ぬ

gistfile1.rb

# from http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association

##### Model #####
class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, through: :appointments

  accepts_nested_attributes_for :appointments
end

class Appointment < ActiveRecord::Base
  belongs_to :physician
  belongs_to :patient
end

class Patient < ActiveRecord::Base
  has_many :appointments
  has_many :physicians, through: :appointments
end

##### Migration #####
create_table "appointments", force: true do |t|
  t.integer  "physician_id",     null: false
  t.integer  "patient_id",       null: false
  t.datetime "appointment_date"
  t.datetime "created_at"
  t.datetime "updated_at"
end

add_index "appointments", ["patient_id"], name: "index_appointments_on_patient_id", using: :btree
add_index "appointments", ["physician_id"], name: "index_appointments_on_physician_id", using: :btree

create_table "patients", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "physicians", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

##### Example #####
describe '' do
  before do
    Patient.create
  end

  it {
    expect  {
      physician = Physician.new
      physician.patients = [Patient.first]
      physician.save!
    }.to_not raise_error

    # 4.1.1 => passed
    # 4.1.2 => expected no Exception, got #<ActiveRecord::StatementInvalid:
    #          Mysql2::Error: Field 'physician_id' doesn't have a default value: 
    #          INSERT INTO `appointments` (`created_at`, `patient_id`, `updated_at`) VALUES ('2015-01-23 04:01:18', 12, '2015-01-23 04:01:18')>
  }
end