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我按照RailsCast #154 on Polymorphic Association设置迁移，如下所示：

# db/migrate/20130719120000_create_comments.rb
class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string :subject
      t.text :message
      t.belongs_to :commentable, polymorphic: true    
      t.timestamps
    end
    add_index :comments, [:commentable_id, :commentable_type]
  end
end

然后模型如下所示：

# app/models/comment.rb
class Comment < ActiveRecord::Base
  attr_accessible :message, :subject
  belongs_to :commentable, polymorphic: true
  counter_culture :commentable # This does not work, read below.
end

一个评论可以设置多个模型，这里是一个例子：

# app/models/product.rb
class Product < ActiveRecord::Base    
  has_many :comments, as: :commentable
  attr_accessible :name
end

这工作得很好。

现在我还使用gem counter_culture（正如您在评论模型中看到的那样），它简单地计算评论的数量。它将计数值存储为关联模型的属性，在本例中为Product。在我将Comment模型切换为多态性之前，集成工作。

当我更改为时counter_culture :commentable，counter_culture :product我最终收到以下错误消息：

NoMethodError: undefined method `product' for #<Comment:0x0000000531cdb0>
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activemodel-3.2.13/lib/active_model/attribute_methods.rb:407:in `method_missing'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activerecord-3.2.13/lib/active_record/attribute_methods.rb:149:in `method_missing'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:301:in `foreign_key_value'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:243:in `change_counter_cache'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:196:in `block (2 levels) in _update_counts_after_create'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:194:in `each'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:194:in `block in _update_counts_after_create'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:186:in `call'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:186:in `_wrap_in_counter_culture_active'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/counter_culture-0.1.12/lib/counter_culture.rb:193:in `_update_counts_after_create'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `_run__2202164684578441182__create__891150410440608527__callbacks'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `__run_callback'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:385:in `_run_create_callbacks'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:81:in `run_callbacks'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activerecord-3.2.13/lib/active_record/callbacks.rb:268:in `create'
    from /home/jjd/.rvm/gems/ruby-1.9.3-p448@demoapp/gems/activerecord-3.2.13/lib/active_record/persistence.rb:348:in `create_or_update'

这基本上是指_update_counts_after_create第192 行。

# Source: https://github.com/magnusvk/counter_culture/blob/master/lib/counter_culture.rb#L192
# called by after_create callback
def _update_counts_after_create
  _wrap_in_counter_culture_active do
    self.class.after_commit_counter_cache.each do |hash|
      # increment counter cache
      change_counter_cache(hash.merge(:increment => true))
    end
  end
end

当我检查hashwhilecounter_culture :commentable设置时，我可以看到必须将:relation其解析为Product模型。

{:relation=>[:commentable], :counter_cache_name=>"comments_count", :column_names=>nil, :foreign_key_values=>nil}

我认为这应该在分配关系的第 29 行中完成。

:relation => relation.is_a?(Enumerable) ? relation : [relation],

如何测试是否relation是多态关联来解析Product模型？

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1 回答 1

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我发现实际上正在进行关于多态性支持的讨论，并且 Dan Maglasang 有fork（最后更新于 2013 年 1 月 16 日，分支于 v.0.1.8）。我检查了它，它对我来说是开箱即用的。
我要求作者重新设置他的分支以更新项目。

于 2013-08-08T08:19:07.053 回答