混淆关联和从控制台获取信息

时间:2016-03-14 22:09:49

标签: ruby-on-rails ruby ruby-on-rails-4 associations

试图从Rails控制台获取信息的困惑

我有以下型号:Article& Comment

对于Article,它是has_many :comments

对于Comment,它是belongs_to :article

让我们假设他们是关联的,并且是功能性的。他们顺便说一句。

这是架构:

ActiveRecord::Schema.define(version: 20160312052519) do

  create_table "articles", force: :cascade do |t|
    t.string   "title"
    t.text     "text"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "comments", force: :cascade do |t|
    t.string   "commenter"
    t.text     "body"
    t.integer  "article_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  add_index "comments", ["article_id"], name: "index_comments_on_article_id"

end

这些是路线:

  resources :articles do 
    resources :comments 
  end

当我运行如下命令时:

Article.first.comments

Comment.first.article

Comment.first.article.title

Comment.first.article.text

Comment.first.commenter

Comment.first.body

他们都工作。

但是当我尝试在Articles上反向运行时:

Article.first.comments.commenter

Article.first.comments.body

Article.all.comments

他们不工作。为什么不?

编辑

这是我运行Article.first.comments.commenter

时得到的结果
Article.first.comments.commenter
  Article Load (23.0ms)  SELECT  "articles".* FROM "articles"  ORDER BY "articles"."id" ASC LIMIT 1
NoMethodError:   Comment Load (0.4ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
undefined method `commenter' for #<Comment::ActiveRecord_Associations_CollectionProxy:0x007fa32b7e3d00>
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activerecord-4.2.5/lib/active_record/relation/delegation.rb:136:in `method_missing'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activerecord-4.2.5/lib/active_record/relation/delegation.rb:99:in `method_missing'
    from (irb):127
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/railties-4.2.5/lib/rails/commands/console.rb:110:in `start'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/railties-4.2.5/lib/rails/commands/console.rb:9:in `start'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:68:in `console'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/railties-4.2.5/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/railties-4.2.5/lib/rails/commands.rb:17:in `<top (required)>'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:274:in `require'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:274:in `block in require'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:240:in `load_dependency'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:274:in `require'
    from /Users/aa/dropbox/beginningRails/guideblogagain/bin/rails:9:in `<top (required)>'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:268:in `load'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:268:in `block in load'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:240:in `load_dependency'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.2.5/lib/active_support/dependencies.rb:268:in `load'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/commands/rails.rb:6:in `call'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/command_wrapper.rb:38:in `call'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:185:in `block in serve'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:156:in `fork'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:156:in `serve'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:131:in `block in run'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:125:in `loop'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application.rb:125:in `run'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/spring-1.6.4/lib/spring/application/boot.rb:18:in `<top (required)>'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/aa/.rbenv/versions/2.1.4/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'

2 个答案:

答案 0 :(得分:1)

Article.first.comments是一个ActiveRecord_Associations_CollectionProxy对象,类似于数组。如果您想获得每条评论的评论者,可以使用map

Article.first.comments.map(&:commenter)

或者您可以获得第一条评论的评论者:

Article.first.comments.first.commenter

同样的事情:

Article.first.comments.body # Won't work

Article.first.comments.map(&:body) # Returns an array (each comment's body)

Article.all.comments # Won't work

Article.all.map(&:comments) # Returns an array (each article's comments)

答案 1 :(得分:1)

运行工作示例Article.first.comments时,请查看实际返回值。你看到了什么?返回是一个对象数组,对吧?所以说Article.first有10 Comments。运行查询时,ActiveRecord将返回10个Comment个对象的数组。

现在让我们来看看你的一个非工作示例。

说,Article.first.comments.commenter。我们再次从Article.first开始,我们知道我们在DB中返回第一个Article对象。

但下一步是考虑Article.first.comments

我们已经说服自己,这是一组Comment个对象,对吗?这就是问题所在 - commenter是单个Comment对象的属性。

但是你没有一个Comment对象 - 你有一个Comment个对象数组,并且没有在数组上定义commenter

Article.first.comments.body问题是一样的。这有意义吗?

您可以在控制台中执行以下操作:

Article.first.comments.each do |comment|  
  puts comment.commenter  
end 

然后您可以获得所有评论者/身体价值观。

使用这种思路,我认为你能够用Article.all.comments说服自己解决这个问题,但如果没有,请随意评论,我们可以说出来。

Here are the docs用于Active Record关系,供参考。干杯!