orbit-basic/app/models/tagging.rb

class Tagging
  include Mongoid::Document
  include Mongoid::Timestamps

  field :to_destroy, default: false

  belongs_to :tag
  belongs_to :taggable, polymorphic: true

  before_destroy :update_taggable_tag_ids

  private

  def update_taggable_tag_ids
    self.taggable.remove_id(self.tag.id)
  end
end