我正在尝试在以前存在的文档中嵌入新文档。创建对象时，根文档如下所示：
class MongoPoll
include Mongoid::Document
embeds_one :region_count, :as => :voteable, :class_name => 'VoteCount'
我添加了一些同一类的新嵌入式文档：
embeds_one :browser_count, :as => :voteable, :class_name => 'VoteCount'
embeds_one :os_count, :as => :voteable, :class_name => 'VoteCount'
VoteCount类看起来像：
class VoteCount
include Mongoid::Document
embedded_in :voteable, :polymorphic => true
当我从头开始创建新文档时，我在before_create处理程序中创建了嵌入式文档，它工作正常：
self.region_count = VoteCount.new
self.browser_count = VoteCount.new
self.os_count = VoteCount.new
但是，为了迁移我的旧文档，我尝试执行相同的任务（poll.os_count = VoteCount.new; poll.save），并且它无声地为os_count失败（但奇怪的是不适用于browser_count）。当我尝试使用rails控制台中的update_attribute或poll['os_count'] = VoteCount.new进行保存时，我得到的堆栈跟踪如下所示：
BSON::InvalidDocument: Cannot serialize an object of class VoteCount into BSON.
from /Library/Ruby/Gems/1.8/gems/bson-1.4.0/lib/../lib/bson/bson_c.rb:24:in `serialize'
from /Library/Ruby/Gems/1.8/gems/bson-1.4.0/lib/../lib/bson/bson_c.rb:24:in `serialize'
from /Library/Ruby/Gems/1.8/gems/mongo-1.4.0/lib/../lib/mongo/collection.rb:426:in `update'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/collections/master.rb:19:in `update'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/collections/retry.rb:29:in `retry_on_connection_failure'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/collections/master.rb:18:in `update'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/collection.rb:149:in `update'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence/operations/update.rb:45:in `persist'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence/modification.rb:25:in `prepare'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:414:in `_run_update_callbacks'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:94:in `send'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:94:in `run_callbacks'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence/modification.rb:24:in `prepare'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:414:in `_run_save_callbacks'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:94:in `send'
from /Library/Ruby/Gems/1.8/gems/activesupport-3.0.9/lib/active_support/callbacks.rb:94:in `run_callbacks'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence/modification.rb:23:in `prepare'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence/operations/update.rb:43:in `persist'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence.rb:86:in `update'
from /Library/Ruby/Gems/1.8/gems/mongoid-2.2.2/lib/mongoid/persistence.rb:151:in `save'
from (irb):5
知道这里可能有什么问题吗？