获取父对象

Question

这是我自己参考模型和它的两个连接表：

class Discourse < ActiveRecord::Base 
    belongs_to :forum 
    belongs_to :user 

    has_many :impressions 

    has_many :discourse_replies 
    has_many :replies, through: :discourse_replies 

    has_many :reply_retorts 
    has_many :retorts, through: :reply_retorts 
end 

class DiscourseReply < ActiveRecord::Base 
    belongs_to :discourse 
    belongs_to :reply, class_name: 'Discourse', foreign_key: 'reply_id' 
end 

class ReplyRetort < ActiveRecord::Base 
    belongs_to :reply 
    belongs_to :retort, class_name: 'Discourse', foreign_key: 'retort_id' 
end 

这似乎运作良好...我可以在铁轨控制台做到这一点：

2.0.0p247 :044 > fd = Discourse.create(title: 'first', body: 'first') 
=> #<Discourse id: 139, user_id: nil, title: "first", body: "first", deleted: nil, delete_date: nil, created_at: "2014-04-07 20:38:06", updated_at: "2014-04-07 20:38:06", forum_id: nil> 

2.0.0p247 :046 > fdr = fd.replies.create(title: 'second relpy to first', body: 'second reply to first')  
=> #<Discourse id: 141, user_id: nil, title: "second relpy to first", body: "second reply to first", deleted: nil, delete_date: nil, created_at: "2014-04-07 20:38:51", updated_at: "2014-04-07 20:38:51", forum_id: nil> 

2.0.0p247 :047 > fdrr = fdr.retorts.create(title: 'a reply to a reply', body: 'a reply to a reply') 
=> #<Discourse id: 142, user_id: nil, title: "a reply to a reply", body: "a reply to a reply", deleted: nil, delete_date: nil, created_at: "2014-04-07 20:39:27", updated_at: "2014-04-07 20:39:27", forum_id: nil> 

2.0.0p247 :048 > fdrrr = fdrr.retorts.create(title: 'a reply to a reply to a reply', body: 'a reply to a reply reply') 
=> #<Discourse id: 143, user_id: nil, title: "a reply to a reply to a reply", body: "a reply to a reply reply", deleted: nil, delete_date: nil, created_at: "2014-04-07 20:39:47", updated_at: "2014-04-07 20:39:47", forum_id: nil> 

2.0.0p247 :050 > fdr.retorts 
=> #<ActiveRecord::Associations::CollectionProxy [#<Discourse id: 142, user_id: nil, title: "a reply to a reply", body: "a reply to a reply", deleted: nil, delete_date: nil, created_at: "2014-04-07 20:39:27", updated_at: "2014-04-07 20:39:27", forum_id: nil>]> 

不过，我需要找出家长协会，但无法弄清楚如何做到这一点：

2.0.0p247 :053 > fdr.discourse # I want this to return the 'fd' instance 
NoMethodError: undefined method `discourse` for #<Discourse:0x00000007080eb0> 

2.0.0p247 :055 > fdrrr.reply # I want this to return the 'fdrr' instance 
NoMethodError: undefined method `reply` for #<Discourse:0x000000070db860> 

2.0.0p247 :055 > fdrrr.parent # I want this to return the 'fdrr' instance 
NoMethodError: undefined method `parent' for #<Discourse:0x0000000672b428> 

2.0.0p247 :055 > fdrrr.parent.try(:id) # I want this to return the 'fdrr' instance 
NoMethodError: undefined method `parent' for #<Discourse:0x0000000672b428> 

没有什么工作！

Answer 1

class Discourse < ActiveRecord::Base 
    def reply 
     DiscourseReply.where(reply_id: id).first.try(:discourse) || 
     ReplyRetort.where(retort_id: id).first.try(:reply) 
    end 

    alias_method :parent, :reply 
    alias_method :discourse, :reply 
end 

但你可能要存储在Discourse模型父的ID，并有可能通过多态（使用STI）Discourse？我觉得如果不查看关联就不能说出对象的类型，这很奇怪。