Monogid HABTM bug - https://jira.mongodb.org/browse/MONGOID-4520

README.md

To run example:

bundle install

bundle exec ruby mongoid_test.rb

Gemfile

gem "minitest", "~> 5.11"

gem "mongoid", "~> 7.0"

Gemfile.lock

GEM
  specs:
    activemodel (5.2.0)
      activesupport (= 5.2.0)
    activesupport (5.2.0)
      concurrent-ruby (~> 1.0, >= 1.0.2)
      i18n (>= 0.7, < 2)
      minitest (~> 5.1)
      tzinfo (~> 1.1)
    bson (4.3.0)
    concurrent-ruby (1.0.5)
    i18n (1.0.1)
      concurrent-ruby (~> 1.0)
    minitest (5.11.3)
    mongo (2.5.3)
      bson (>= 4.3.0, < 5.0.0)
    mongoid (7.0.1)
      activemodel (>= 5.1, < 6.0.0)
      mongo (>= 2.5.1, < 3.0.0)
    thread_safe (0.3.6)
    tzinfo (1.2.5)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  minitest (~> 5.11)
  mongoid (~> 7.0)

BUNDLED WITH
   1.16.3

mongoid.yml

test:
  clients:
    default:
      database: mongoid_test
      hosts:
        - dockerhost:27017

mongoid_test.rb

require 'rubygems'
require 'bundler/setup'

require 'mongoid'
require 'minitest/autorun'

Mongoid.load!('mongoid.yml', 'test')

class Band
  include Mongoid::Document
  has_and_belongs_to_many :tags
end

class Tag
  include Mongoid::Document
  has_and_belongs_to_many :bands
end

class TestHABTM < Minitest::Test
  def setup
    Band.delete_all
    Tag.delete_all

    @band = Band.new.save
    @tag = Tag.new
  end

  # This example fails
  def test_band_habtm_is_persisted_automatically_after_tag_is_saved
    @tag.bands = Band.all
    @tag.save!

    assert_equal [@tag], Band.first.tags
  end

  def test_multiple_bands_are_persisted_automatically_after_tag_is_saved
    Band.new.save
    @tag.bands = Band.all
    @tag.save!

    assert_equal [@tag], Band.all[0].tags
    assert_equal [@tag], Band.all[1].tags
  end

  def test_band_habtm_is_persisted_after_tag_is_saved
    @tag.save!
    @tag.bands = Band.all

    assert_equal [@tag], Band.first.tags
  end
end