日志中的未知SQL平均查询Rails

Question

全部都是，

我刚刚对SQL平均查询有个疑问。

我有一个帖子模型，它有很多评级。某个特定帖子的所有评分都加起来，并除以平均评分数。所以这个计算是在后期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，其作者，姓名和评分均来自帖子评分数据库列。

我的问题是，为什么rails在这些值已被保存在数据库中时执行所有这些SQL平均查询？

我的代码如下：

在控制台

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 

帖子/ 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指数行动（我有三编预先加载，但平均疑问仍然存在）

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

SQL日志，有很多查询

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就骂。 非常感谢，安迪

Answer 1

你有一个称为评价的方法，这个方法在你的模型中覆盖了访问器，所以即使你有一个db列'评级'，当你询问model.rating时也不会被调用，这个方法将会再次重复评级。此外，该方法不保存（只需设置一个属性不会保留到数据库，您需要调用保存或类似）。所以如果你想把价值保存在评价中，我会摆脱这种方法。

你应该真的这样做时，该职位添加了评分或更改 - 在这一点上，调用update_rating或什么（见下文），然后确保你打电话post.save之后（使之成为before_save回调，或致电self.update_attribute或self.save explitly在update_rating） - 将更改保存到数据库，然后你就可以在视图中使用

<%= post.rating %> 

或其他地方作为缓存的平均评分值。

在Post模型：

before_save :update_rating 

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

Answer 2

您的循环在您的文章。在每个循环中，您拨打：

post.rating 

这在您的AVG SQL查询的转火：

self.ratings.average(:rating) 

它不会被保存，它只是计算让你 '如果post.rating =' 工作

Answer 3

根据我的经验，我发现，@肯尼 - 格兰特的update_rating方法实际上并不包括新计算并持续更新的等级增加时的平均收视率。在这个阶段

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 

这确保了新添加的记录显示，自ratings是到数据库的调用，而不是在内存：

这很适合我。 update_columns可让您重写记录，而不会导致触发其所有回叫的额外动作save。在写入列时使用update_rating方法还会设置内存中评级属性，除非您之后还重新加载了模型，否则update_columns将不会执行此操作。