Ruby 附加后未运行的保存回调

我在尝试使用

添加关系时遇到问题，您缺少关系，has n，：cuts

class Color
  include DataMapper::Resource

  property :id,   Serial
  property :name, String

  has n,      :cuts
  has n,      :fabrics, :through => Resource
end

想详细说明一下吗？你是说模型不见了？（事实并非如此；我在帖子中将其排除在外）或者我需要在我的颜色中添加一个或多个剪切以使其正确工作？为什么会产生这种行为？
class Color
  include DataMapper::Resource

  property :id,   Serial
  property :name, String

  has n,      :cut
  has n,      :fabrics, :through => Resource
end

yellow = Color.new(:name => "yellow")
red = Color.new(:name => "red")

f = Fabric.create(:name => "tricot", :fixed_color => false)

f.colors << red
f.save
f.colors << yellow
f.save
puts f.colors.size
=> 0
=> 2

f.colors = f.colors + [red]
f.save
f.colors = f.colors + [yellow]
f.save
puts f.colors.size
=> 0
=> 1
=> 2
=> 2

class Color
  include DataMapper::Resource

  property :id,   Serial
  property :name, String

  has n,      :cuts
  has n,      :fabrics, :through => Resource
end