def setup_callbacks_for_habtm(join_tables)
      @model_class.send :attr_accessor, :paper_trail_habtm
      @model_class.class_attribute :paper_trail_save_join_tables
      @model_class.paper_trail_save_join_tables = Array.wrap(join_tables)
      habtm_assocs_not_skipped.each(&method(:setup_habtm_change_callbacks))
    end
def setup_callbacks_for_habtm(join_tables)
      @model_class.send :attr_accessor, :paper_trail_habtm
      @model_class.class_attribute :paper_trail_save_join_tables
      @model_class.paper_trail_save_join_tables = Array.wrap(join_tables)
      habtm_assocs_not_skipped.each(&method(:setup_habtm_change_callbacks))
    end
Adds callbacks to record changes to habtm associations such that on save
 the previous version of the association (if changed) can be reconstructed.