require 'rubygems'
require 'data_mapper'

DataMapper.setup :default, "mysql://root:123456@localhost/cc"

class Url
  include DataMapper::Resource

  property :id, Serial
  property :original, String,:length => 255
  belongs_to :link
end


class Link
  include DataMapper::Resource

  property :id, Serial
  property :identifier, String
  property :created_at, Date
  has 1, :url
  has n, :visits

end

class Visit
  include DataMapper::Resource

  property :id, Serial
  property :created_at, Date
  property :ip, String
  property :contry, String
  belongs_to :link

end

DataMapper.auto_migrate!
DataMapper.finalize

url = Url.create(:original => "http://")
p url

?

#<Url @id=nil @original="http://" @link_id=nil>

?

在创建关联类的对象时，没有进行save的时候，@id的值正常就是nil，但是有些时候需要它有值

解决办法是在belongs_to后面加上,:required => false

?

require 'rubygems'
require 'data_mapper'

DataMapper.setup :default, "mysql://root:123456@localhost/cc"

class Url
  include DataMapper::Resource

  property :id, Serial
  property :original, String,:length => 255
  belongs_to :link,:required => false
end


class Link
  include DataMapper::Resource

  property :id, Serial
  property :identifier, String
  property :created_at, Date
  has 1, :url
  has n, :visits

end

class Visit
  include DataMapper::Resource

  property :id, Serial
  property :created_at, Date
  property :ip, String
  property :contry, String
  belongs_to :link,:required => false

end

DataMapper.auto_migrate!
DataMapper.finalize

url = Url.create(:original => "http://")
p url

?#<Url @id=1 @original="http://" @link_id=nil>

?