了解Rails ActiveRecord的“单一模型"自我加入

问题描述

我很难理解如何在Rails中实现单个模型自联接."ActiveRecord关联指南"部分2.10 简要说明了自助加入"，但没有提供足够的信息，并且每个示例或与此相关的示例均引用了不是单个模型自联接的Friendly Friend Railcast示例，如《铁路指南》第2.10节.

I'm having a hard time understanding how to implement a single model self-join in Rails. The Guide to ActiveRecord Associations section 2.10 briefly explains Self-Joins but doesn't offer enough information, and every example or post about this such references the Friendly Friend Railcast example that isn't a single model self join, as described in the Rails Guide section 2.10.

这个想法是一个具有has_many和belongs_to本身的模型，不需要为关系建立单独的表.我看到需要一个单独的表的唯一原因是，您是否希望该关系包含比该关系更多的信息.例如最好的朋友"，几乎不认识他们"

The idea is a model that has_many and belongs_to itself, without needing a separate table for the relationship. The only reason I see for needing a separate table is if you want the relationship to contain more information than just the relationship. e.g. "best friends", "barely know them"

我有一个简单的Post模式:

I have a simple Post schema:

create_table "posts", :force => true do |t|
    t.datetime "posted"
    t.string   "nick"
    t.string   "title"
    t.text     "content"
    t.integer  "parent_post_id"
    t.datetime "created_at",     :null => false
    t.datetime "updated_at",     :null => false
end

parent_post_id是对其他Post post_id的自引用.posts.rb模型具有定义的关系:

The parent_post_id is a self-reference to other Post post_id's. The posts.rb model has the relationship defined:

class Post < ActiveRecord::Base
  has_many :replies, :class_name => "Post"
  belongs_to :parent_post, :class_name => "Post",
    :foreign_key => "parent_post_id"
end

在Controller或View中，我希望能够执行以下操作:

In the Controller or View I'm hoping to be able to do something like this:

@posts.each do |post|
  ...
  @replies = post.replies
  @replies.each do |reply|
    ...
  end
end

或找到帖子的父级:

@parent_post = post.parent_post

这可能是一些语法误解.因此，在此先感谢任何可以对我有所了解的人.我浏览了所有SO和博客文章，没有人尝试使用《指南》中介绍的单一模型自引用自联接方法.

This may all be some syntax mis-understanding. So thanks in advance to anyone who can slap some sense into me. I've looked through every SO and blog post out there and none try the single model self-referential self-join method described in the Guide.

为任何提供解释而未指向使用单独关系表的友好朋友"示例的人提供积分.

Points for anyone offering an explanation that doesn't point to the Friendly Friend example that uses a separate relationship table.

推荐答案

我缺少"parent_post_id"的has_many外键.设置好后，post.replies将通过parent_post_id引用其他Post实例.

I was missing the has_many foreign key to "parent_post_id". Once its set, the post.replies will reference other Post instances by parent_post_id.

posts.rb模型具有定义的关系:

The posts.rb model has the relationship defined:

class Post < ActiveRecord::Base
  has_many :replies, :class_name => "Post",
    :foreign_key => "parent_post_id"
  belongs_to :parent_post, :class_name => "Post",
    :foreign_key => "parent_post_id"
end

我现在可以创建帖子，为另一个帖子分配一个parent_post_id，然后再获取所有对任何父帖子进行回复的帖子.

I can now create Posts, assign a parent_post_id to a different Post, then later get all Posts that are replies to any parent Post.

这篇关于了解Rails ActiveRecord的“单一模型"自我加入的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！