Track habtm changes on active record

track_habtm_changes.rb

# See https://groups.google.com/forum/#!topic/rubyonrails-core/Lb9rBkZnZSo
module TrackHabtmChanges
  def self.included(model)
    model.after_initialize :track_habtm_initial_state
    model.after_save :track_habtm_initial_state
  end

  def changes
    super.merge(habtm_changes)
  end

  def changed_attributes
    changed_attributes = super

    habtm_changes.each do |key, (_, new)|
      changed_attributes[key] = new
    end

    changed_attributes
  end

  protected

  def track_habtm_initial_state
    @habtm_initial_state = {}

    self.class.reflect_on_all_associations.each do |association|
      next unless association.macro == :has_and_belongs_to_many

      method = "#{association.name.to_s.singularize}_ids"

      @habtm_initial_state[method] = send(method)
    end
  end

  def habtm_changes
    @habtm_initial_state.inject({}) do |habtm_changes, (association, initial_state)|
      current_state = send(association)

      if initial_state != current_state
        habtm_changes[association] = [initial_state, current_state]
      end

      habtm_changes
    end
  end
end

Warning: It is slow when you fetch hundreds of objects (n + 1)

Instead of this:

self.class.reflect_on_all_associations.each do |association|
  next unless association.macro == :has_and_belongs_to_many

you can do this:

self.class.reflect_on_all_associations(:has_and_belongs_to_many).each do |association|
...