#!/usr/bin/env ruby
require 'mongoid'

DB_NAME = 'test_find_or_initialize_by'

class Band
  include Mongoid::Document
  field :active, type: Boolean
  embeds_many :members, inverse_of: :band
  validates_presence_of :members, if: lambda { |b| b.active }
end

class Member
  include Mongoid::Document
  field :name, type: String
  embeds_many :instruments, inverse_of: :member
  embedded_in :band, inverse_of: :members

  validates_presence_of :band # Comment this out and the bug goes away!
end

class Instrument
  include Mongoid::Document
  field :name, type: String
  embedded_in :member, inverse_of: :instruments
end

Mongoid.load_configuration(sessions: { default: { database: DB_NAME, hosts: ['localhost'], options: {} } })

b = Band.create!(active: false)
alice = b.members.create(name: 'alice')
b.active = true
trumpet = alice.instruments.build(name: 'trumpet')
alice.save!
# You now have two "trumpet" documents in the "alice" document