如何按最後評論的日期排序，並按上次創建的順序排序？

Question

我有一個後型號：

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 

和評論型號：

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

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

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

現在我想知道，如何按最後一條評論的日期排序並按上次創建的順序排序？

幾乎StackOverflow是如何做的。例如，如果問題得到回覆，則將其置於頂部，如果回答了其他問題，則將該問題置於頂部，依此類推。否則，問題將按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

首先，你需要另外一個DateTime列添加到您的文章。我們稱之爲content_changed_at或類似的東西。在一篇新文章中，它的值爲created_at。然後，每當評論發佈時更新它。當您需要顯示帖子時，您只需按此欄對其進行排序。

這是未經測試的代碼，但它應該給你的想法。

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 

Answer 2

如果我是你，我會一個last_comment_at補充：時間字段來安置自己的模式，一定要更新每當創建的評價是日期時間。

這種方式，您可以簡單地使用Post.order('last_comment_at, created_at DESC')

更新 - 回覆您的留言詢問你需要遷移，以下應爲您生成遷移：

rails generate migration add_last_comment_at_to_posts last_comment_at:datetime 

正如其他有請注意，您可以在評論模型中添加after_save調用，以更新Post模型中的last_comment_at字段。我會用這樣的：

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