Question

晚上好，

我刚才有一个关于SQL平均查询的快速问题。

我有一个有很多评级的帖子模型。特定帖子的所有评级都会加起来并除以平均评分的数量。因此，此计算在post模型中声明并保存在数据库中。在我的索引视图中，我列出了所有帖子，包括作者，姓名和评级，这些帖子来自后评级数据库列。

我的问题是，为什么当这些值保存在数据库中时，rails会执行所有这些SQL平均查询？

我的代码如下：

在控制台中

Post Load (1.0ms)  SELECT `posts`.* FROM `posts` ORDER BY posts.created_at ASC LIMIT 1
 => #<Post id: 48, user_id: 5, song_name: "Enter Sandman", song: "ewwe wer ere rr erew rewr r erw erw rewedwde", created_at: "2012-08-16 13:35:32", updated_at: "2012-08-22 21:11:34", rating: 5.0, ratings_count: 2>

Post.rb

def rating
    if self.ratings.any?
      self.rating = self.ratings.average(:rating)
    end
  end

文章/ index.html.erb

<legend>Music library</legend>  

    <%= will_paginate @paginate_posts %>

            <% if @paginate_posts.any? %>
                <% @paginate_posts.each do |post| %>
            <h4><%= link_to post.song_name, post %></h4>
                Author: <%= link_to post.user.name, "#" %><br/>
                    <% if post.rating == nil %>
                        No one has rated this yet<br/>
                    <% else %>
                        Rating: <%= post.rating %>/10<br/>
                    <% end %>
                <br/>
        <% end %>
<% end %>

<%= will_paginate @paginate_posts %>

posts_controller.rb索引操作（我尝试过急切加载，但avg查询仍在那里）

def index
    @paginate_posts = Post.paginate(page: params[:page], per_page: 10).includes(:user).search(params[:search])
  end

包含大量查询的SQL日志

Started GET "/" for 127.0.0.1 at 2012-08-22 22:51:38 +0100
Processing by PostsController#index as HTML
  Post Load (0.3ms)  SELECT `posts`.* FROM `posts` ORDER BY posts.created_at DESC LIMIT 10 OFFSET 0
  User Load (0.3ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` IN (5)
   (0.3ms)  SELECT COUNT(*) FROM `posts` 
  Rendered posts/_copy.html.erb (0.1ms)
   (0.3ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 63
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 63
   (0.3ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 62
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 62
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 61
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 61
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 60
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 60
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 59
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 59
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 58
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 58
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 57
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 57
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 56
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 56
   (0.2ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 55
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 55
   (0.3ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 54
  CACHE (0.0ms)  SELECT AVG(`ratings`.`rating`) AS avg_id FROM `ratings` WHERE `ratings`.`post_id` = 54
  Rendered posts/index.html.erb within layouts/application (60.8ms)
  User Load (0.4ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 5 LIMIT 1
  Rendered layouts/_footer.html.erb (0.1ms)
Completed 200 OK in 2012ms (Views: 1871.7ms | ActiveRecord: 33.1ms)

像往常一样如果你需要更多的代码只是喊叫。非常感谢，安迪

Answer 1

你在post中有一个名为rating的方法，它会覆盖你模型上的访问器，所以即使你有一个db列'rating'，当你要求model.rating时它也不会被调用，方法将是，这将再次迭代收视率。另外该方法没有保存（只是设置属性不会持久存储到db，需要调用save或类似）。因此，如果您想将值保存在评级中，我将摆脱该方法。

当帖子添加或更改评级时，你应该真的这样做 - 此时，调用update_rating或其他东西（见下文），然后确保你之后调用post.save（或者让它成为before_save回调，或者在update_rating中explitly调用self.update_attribute或self.save） - 这会将更改保存到db，然后你可以使用

<%= post.rating %>

在视图或其他任何位置作为缓存平均评级值。

帖子模型中的

：

before_save :update_rating

def update_rating
  if self.ratings.any?
    self.rating = self.ratings.average(:rating)
  end
end

Answer 2

你的帖子循环。在每个循环中，您调用：

post.rating

反过来触发你的AVG SQL查询：

self.ratings.average(:rating)

它没有保存，它只是计算所以你的'if post.rating ='工作

Answer 3

根据我的经验，我发现在计算和持续更新评级时，@ Kenny-Grant的update_rating方法实际上不会在平均值中包含新添加的评分。

这对我有用：

after_save :update_rating!

def update_rating
  self.rating = ratings.average(:rating) || 0.0
end

def update_rating!
  self.update_columns(rating: update_score)
end

这确保显示新添加的记录，因为ratings是对DB的调用，而不是在此阶段的内存中。 update_columns允许您重写记录，而不会导致额外的save操作，从而触发所有回调。在编写列时使用update_rating方法也会设置内存中的评级属性，除非您之后还重新加载模型，否则update_columns不会执行此操作。

日志中的未知SQL平均查询Rails

3 个答案: