Question

有文章和评论。

目标：列出所有文章最后发布的所有评论。

例如：

文章＃1有3条评论

文章＃2有1条评论

第3条没有评论


目标：获取最后在文章＃1，＃2
上创建的2条评论

class Article < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :article, counter_cache: true

 def self.last_comment
   # list all comments which are created last on any article
 end
end


class CommentsController < ApplicationController

def last_comments
  @last_comments = Comment.last_comment
end

end

Answer 1

以下是你的工作，但在我看来并不是一个好的解决方案：

class Article < ActiveRecord::Base
  has_many :comments
  def last_comment
    comments.last
  end
end


class Comment < ActiveRecord::Base
  belongs_to :article, counter_cache: true

 def self.last_comment
   Article.all.each_with_object([]) { |a, last_comments| last_comments << a.last_comment }
 end
end

现在你可以得到所有最后的评论：

Comment.last_comment

特定文章的最后评论：

@article.last_comment

Answer 2

执行此操作的一种方法是在文章上缓存last_comment_id。

class Article < ActiveRecord::Base
  has_many :comments, after_add: :update_last_comment, after_remove: :update_last_comment
  has_one :last_comment, class_name: "Comment"

  private

  # could process this as an asynchronous job
  def update_last_comment
    self.last_comment = comments.order(created_at: :desc).first
    self.save!
  end
end

class Comment < ActiveRecord::Base
  belongs_to :article, counter_cache: true

  def self.last_comment
    joins(:article).
      where("id = articles.last_comment_id")
  end
end

after_add / after_remove是ActiveRecord关联回调，记录为here。

获取最后的评论

2 个答案: