def each_descendant(&block)
      children.each do |child_model|
        block.call(child_model)
        child_model.each_descendant(&block) if child_model.respond_to?(:each_descendant)
      end
    end
def each_descendant(&block)
      children.each do |child_model|
        block.call(child_model)
        child_model.each_descendant(&block) if child_model.respond_to?(:each_descendant)
      end
    end
Executes the given code block with every model that is a child of this model.