A has_many Bs 其中 B 没有主键

我有型号 A 和 B； A has_many B，并且 B 属于 A。到目前为止，一切都很好，除了我指定 B 没有主键。我不打算修改或删除单个 B 行，并且我预计会有数百万到数十亿的 B 行，因此省略主键将非常方便，节省空间。

创建 B 表的迁移如下所示：

class CreateBs < ActiveRecord::Migration
  def change
    create_table :bs, {:id => false} do |t|
      # … rest of fields …
    end
  end
end

不幸的是，ActiveRecord 不同意；尝试创建一个 A（没错！）会导致：

1.9.3p194 :001 > A.create!
   (0.3ms)  BEGIN
   (0.1ms)  ROLLBACK
ActiveRecord::UnknownPrimaryKey: ActiveRecord::UnknownPrimaryKey
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/reflection.rb:366:in `primary_key'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/reflection.rb:216:in `association_primary_key'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/has_many_association.rb:104:in `foreign_key_present?'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/association.rb:165:in `find_target?'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/collection_association.rb:332:in `load_target'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/collection_proxy.rb:44:in `load_target'
…

如果你捕获了异常，它message states:

"Unknown primary key for table bs in model B." 

（这是因为 B 没有主键。）

我希望没有这个问题！有什么办法吗？

罪魁祸首原来是问题中遗漏的一个细节——当然——也是我工作记忆中遗漏的：

class A < ActiveRecord::Base
  has_many :bs

  validates :bs, :presence => true
end

尽管我当时没有想到这一点，但除了许多其他验证之外，还验证了bs。如果你仔细观察完整的回溯——我在最初的问题中“很有帮助”地截断了它，你会看到：

ActiveRecord::UnknownPrimaryKey: ActiveRecord::UnknownPrimaryKey
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/reflection.rb:366:in `primary_key'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/reflection.rb:216:in `association_primary_key'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/has_many_association.rb:104:in `foreign_key_present?'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/association.rb:165:in `find_target?'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/collection_association.rb:332:in `load_target'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/collection_proxy.rb:44:in `load_target'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activerecord-3.2.8/lib/active_record/associations/collection_proxy.rb:87:in `method_missing'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activemodel-3.2.8/lib/active_model/errors.rb:255:in `block in add_on_blank'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activemodel-3.2.8/lib/active_model/errors.rb:253:in `each'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activemodel-3.2.8/lib/active_model/errors.rb:253:in `add_on_blank'
        from /Users/annelicuss/.rvm/gems/ruby-1.9.3-p194/gems/activemodel-3.2.8/lib/active_model/validations/presence.rb:8:in `validate'
-------------------------------------------------------------------------------------------------^^^^^^^^^^^^^^^^^^^^^^^

错误发生在active_model/validations/presence.rb！消除这个限制就足以让我们继续前进。它无法找到验证的目标，因为它尝试使用主键来执行此操作，因此失败了。