class User < ActiveRecord::Base
  has_many :products
  # <snip>
end

和

class Product < ActiveRecord::Base
  belongs_to :user
  # <snip>
end

问题是产品是在用户注册之前创建的.也就是说,完全可以调用Product.create,并将其设置为nil.尽管如此,您可以想像,Rails不支持这一点：

> Product.create!
   (0.3ms)  SELECT COUNT(*) FROM "products" WHERE "products"."type" IN ('Product')
   (0.1ms)  begin transaction
   (0.1ms)  rollback transaction
ActiveRecord::RecordInvalid: Validation Failed: User can't be blank
    from ~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/validations.rb:56:in `save!'

我已经想到了一堆kludgey的解决方法,最有吸引力的是有一个NullUser子类化用户并使用它来创建产品.但这似乎还是一个黑客.这是什么Rails方式？

谢谢.

相关迁移：

class AddUseridToProducts < ActiveRecord::Migration
  def change
    add_column :products,:user_id,:integer
  end
end

然后：

class Changeuseridtobeoptionalforproducts < ActiveRecord::Migration
  def change
    change_column :products,:integer,null: true
  end
end