如何按最后评论的日期排序，并按最后创建的日期排序？

Question

我有一个Post模型：

class Post < ActiveRecord::Base
  attr_accessible :title, :content, :tag_names

  belongs_to :user

  has_many :comments, :dependent => :destroy
end

  belongs_to :post, :counter_cache => true
  belongs_to :user
end

和评论模型：

class Comment < ActiveRecord::Base
  attr_accessible :content, :user_id

  belongs_to :post, :counter_cache => true
  belongs_to :user
end

现在，我知道如何按日期排序： created_at ASC ， created_at DESC 。

现在，我想知道，如何按最后评论的日期排序，并按最后创建的排序？

StackOverflow的功能差不多。 例如，如果一个问题得到答复，则该问题位于顶部，如果其他问题得到答复，则该问题位于顶部，依此类推。 否则，问题按ASC排序。

生成的SQL：

Started POST "/posts/30/comments" for 127.0.0.1 at 2012-03-12 11:59:32 +0800
Processing by CommentsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"0zGpin7T/1aH/JBomGiqsEYCIgQ04HQWHZ/bIjm7WKs=", "comment"=>{"content"=>"sfsfwfewsdesd"}, "commit"=>"Create Comment", "post_id"=>"30"}
  Post Load (0.3ms)  SELECT "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT 1  [["id", "30"]]
  User Load (0.6ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 2 LIMIT 1
   (0.2ms)  begin transaction
  SQL (1.1ms)  INSERT INTO "comments" ("content", "created_at", "post_id", "total_votes", "updated_at", "user_id") VALUES (?, ?, ?, ?, ?, ?)  [["content", "sfsfwfewsdesd"], ["created_at", Mon, 12 Mar 2012 03:59:32 UTC +00:00], ["post_id", 30], ["total_votes", 0], ["updated_at", Mon, 12 Mar 2012 03:59:32 UTC +00:00], ["user_id", 2]]
  Post Load (0.4ms)  SELECT "posts".* FROM "posts" WHERE "posts"."id" = 30 LIMIT 1
  SQL (0.5ms)  UPDATE "posts" SET "comments_count" = COALESCE("comments_count", 0) + 1 WHERE "posts"."id" = 30
   (130.3ms)  commit transaction
Redirected to http://localhost:3000/posts/30
Completed 302 Found in 156ms (ActiveRecord: 133.4ms) 

Answer 1

如果您是我，我会在您的Post模型中添加一个last_comment_at：datetime字段，并确保在创建注释时更新该datetime。

这样，您可以简单地使用Post.order('last_comment_at, created_at DESC')

更新-在回答您有关所需迁移的评论时，以下内容将为您生成迁移：

rails generate migration add_last_comment_at_to_posts last_comment_at:datetime

正如其他人指出的那样，您可以在Comment模型中添加一个after_save调用，以更新Post模型中的last_comment_at字段。 我会用这样的东西：

class Comment < ActiveRecord::Base
  belongs_to :post

  after_save :update_post_last_comment_at

  private

  def update_post_last_comment_at
    self.post.touch(:last_comment_at) if self.post
  end
end

Answer 2

首先，您需要在帖子中添加另一个datetime列。 我们称它为content_changed_at或类似名称。 在新帖子中，其值为created_at 。 然后，每当有评论发布时，您就对其进行更新。 当您需要显示帖子时，只需按此列对其进行排序。

这是未经测试的代码，但是它可以带给您想法。

class Post < ActiveRecord::Base
  attr_accessible :title, :content, :tag_names

  before_create :init_sort_column

  belongs_to :user

  has_many :comments, :dependent => :destroy

  private 
  def init_sort_column
    self.content_changed_at = self.created_at || Time.now
  end
end

  belongs_to :post, :counter_cache => true
  belongs_to :user
end

class Comment < ActiveRecord::Base
  attr_accessible :content, :user_id

  after_create :update_parent_sort_column

  belongs_to :post, :counter_cache => true
  belongs_to :user

  private
  def update_parent_sort_column
    if post
      post.content_changed_at = self.created_at
    end
  end
end