对关联和从控制台获取信息的困惑

Question

对尝试从 Rails 控制台获取信息感到困惑

我有以下型号：Article & Comment。

对于Article，它是has_many :comments

对于Comment，它是belongs_to :article

让我们假设它们是关联的并且是功能性的。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

它们都有效。

但是当我尝试在 Articles 上反向运行时，例如：

Article.first.comments.commenter

Article.first.comments.body

Article.all.comments

它们不起作用。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'

Answer 1

Article.first.comments 是一个ActiveRecord_Associations_CollectionProxy 对象，类似于数组。如果您想为每条评论获取评论者，您可以使用map

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

或者你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)

Answer 2

当您运行工作示例Article.first.comments 时，请查看实际返回值。你看到了什么？返回的是一个对象数组，对吧？所以说Article.first 有10 个Comments。当您运行查询时，ActiveRecord 将返回一个包含 10 个Comment 对象的数组。

现在让我们看一下您的一个无效示例。

说，Article.first.comments.commenter。我们再次从Article.first 开始，我们知道它会返回我们数据库中的第一个Article 对象。

但下一步是考虑Article.first.comments。

我们已经说服自己这是Comment 对象的数组，对吧？这就是问题所在——commenter 是单个Comment 对象的属性。

但是您没有单个 Comment 对象 - 您有一个 Comment 对象数组，并且 commenter 未在数组上定义。

Article.first.comments.body 的问题是一样的。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/正文值。

使用这种思路，我认为您将能够说服自己对Article.all.comments 的问题，但如果不是，请随时发表评论，我们可以讨论一下。

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