has_many 多态的多态

User has_many Categories
Category has_many Posts
当Category和Post同为多态时。这样

class User < ActiveRecord::Base
  has_many :categories, :as => :owner #owner_id owner_type
  has_many :posts, :through => :categories
end
class Category < ActiveRecord::Base
  has_many :posts, :as => :postable #postable_id, postable_type
  belongs_to :user, :polymorphic => true
end
class Post < ActiveRecord::Base
  belongs_to :postable, :polymorphic => true
end

当调用user.posts时就会报错
错误原因

  Unknown column 'categories.postable_id' in 'on clause': SELECT posts.* FROM posts INNER JOIN categories ON posts.id = categories.postable_id WHERE ((categories.owner_id = 1) AND (categories.owner_type = 'User'))

红色部分期望的应该是posts.postable_id = categories.id，这里正好相反了
而将代码改为

class User < ActiveRecord::Base
  has_many :categories, :foreign_key => "owner_id", :conditions => "owner_type = 'User'" #owner_id owner_type
  has_many :posts, :through => :categories
end

就可以正常使用了。
y?