验证外键时 Accepts_nested_attributes_for 出现问题

我正在使用 Ruby on Rails v3.2.2。我想解决使用时与外键验证相关的问题accepts_nested_attributes_for and validates_associated回报率方法。也就是说，我有以下模型类：

class Article < ActiveRecord::Base
  has_many :category_associations, :foreign_key => 'category_id'

  accepts_nested_attributes_for :category_associations, :reject_if => lambda { |attributes| attributes[:category_id].blank? }
  validates_associated :category_associations
end

class CategoryAssociation < ActiveRecord::Base
  belongs_to :article, :foreign_key => 'article_id'
  belongs_to :category, :foreign_key => 'category_id'

  validates :article_id, :presence => true
  validates :category_id, :presence => true
end

...我有以下控制器操作：

class ArticlesController < ApplicationController
  def new
    @article = Article.new
    5.times { @article.category_associations.build }

    # ...
  end

 def create
   @article = Article.new(params[:article])

   if @article.save
     # ...
   else
     # ...
   end
 end
end

通过上面的代码（“灵感”来自嵌套模型表格第 1 部分 http://railscasts.com/episodes/196-nested-model-form-part-1?view=asciicast铁轨铸造）我的目的是在创建文章时存储类别关联 (note：类别对象已存在于数据库中；就我而言，我只想存储创建类别关联）。但是，当我从相关视图文件提交相关表单时，出现以下错误（我正在记录错误消息）：

{:"category_associations.article_id"=>["can't be blank"], :category_associations=>["is invalid"]}

为什么会发生这样的事validates_associated seems https://stackoverflow.com/questions/5176510/validates-associated-not-checking-existence-of-associations#comment5826558_5176607运行该方法article.category_association.valid?但前提是article.category_association.article_id is not nil？如何解决存在验证的问题article_id外键？

但是，如果我注释掉validates :article_id, :presence => true in the CategoryAssociation模型类，它按预期工作，但是好像是not正确的做法not验证外键 https://stackoverflow.com/questions/13345250/is-it-really-needed-to-validate-foreign-keys/13346791#13346791.

如果我注释掉validates_associated :category_associations in the Article模型类，我仍然收到错误：

{:"category_associations.article_id"=>["can't be blank"]}

Use inverse_of链接关联，然后验证关联对象是否存在，而不是实际外键是否存在。

示例来自the docs http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html:

class Member < ActiveRecord::Base
  has_many :posts, inverse_of: :member
  accepts_nested_attributes_for :posts
end

class Post < ActiveRecord::Base
  belongs_to :member, inverse_of: :posts
  validates_presence_of :member
end