为什么我的数据映射器验证不起作用?

此示例无法验证是否存在标题:

require 'rubygems'
require 'dm-core'
require 'dm-migrations'
require 'dm-validations'

db_name = 'rvtest.sqlite'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite://' + Dir.pwd + '/' + db_name)

class Task
  include DataMapper::Resource
  property :id,         Serial
  property :title,      String

  validates_presence_of :title
end

DataMapper.finalize
DataMapper.auto_upgrade!

require 'rubygems'
require 'dm-core'
require 'dm-migrations'
require 'dm-validations'

db_name = 'rvtest.sqlite'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite://' + Dir.pwd + '/' + db_name)

class Task
  include DataMapper::Resource
  property :id,         Serial
  property :title,      String, :required => true

end

DataMapper.finalize
DataMapper.auto_upgrade!

为什么这些工作方式不同?如果我进入irb,需要这个文件并键入 Task.new.save! 第一个回来了 true 第二个 false .