轨道3,有许多:通过和<<运算符

将应用程序转换为Rails3。遇到了这个缺失的语法糖。

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :group
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
end

在Rails 3中我不能再这样做了:

$ group = Group.first
$ user = User.first
$ group.users << user
ActiveRecord::StatementInvalid: Mysql2::Error: Column 'user_id' cannot 
be null: INSERT INTO `memberships` (`admin`, `created_at`, 
`group_id`, `leader`, `updated_at`, `user_id`) VALUES 
(0, '2010-11-03 18:31:33', 5, 0, '2010-11-03 18:31:33', NULL)

去哪了 << 走吧?它被什么东西取代了吗?