请看Rails里的Magic Column Names

这次就是讲用_count字段来缓存has_many的计数

看Project和Task的例子:

<h1>Projects</h1>

<table>
<% for project in @projects %>
  <tr>
    <td><%= link_to project.name, poject_path(project) %></td>
    <td><small>(<%= pluralize project.tasks.size, 'task' %>)</small></td>
  </tr>
<% end %>
</table>

上面的页面代码对所有的@projects显示tasks.size，看下log:

SQL (0.006385)  SELECT count(*) AS count_all FROM tasks WHERE (tasks.project_id = 326)
SQL (0.000220)  SELECT count(*) AS count_all FROM tasks WHERE (tasks.project_id = 327)
SQL (0.000383)  SELECT count(*) AS count_all FROM tasks WHERE (tasks.project_id = 328)
SQL (0.000197)  SELECT count(*) AS count_all FROM tasks WHERE (tasks.project_id = 329)
SQL (0.000215)  SELECT count(*) AS count_all FROM tasks WHERE (tasks.project_id = 330)

上面显示了对每个project都使用SQL来count tasks，我们采用eager loading看看能否改进性能:

class ProjectsController < ApplicationController
  def index
    @projects = Project.find(:all, :include => :tasks)
  end
end

再来看看log:

Project Lood Incluing Associations (0.000954)  SELECT projects.'id' AS t0_r0, projects.'name' AS t0_r1, tasks.'id'
AS t1_r0, tasks.'name' AS t1_r1, tasks.'project_id' AS t1_r2 FROM projects LEFT OUTER JOIN tasks ON tasks.project
_id = projects.id

我们看到，使用eager loading确实只用一条SQL语句就完成工作，但是缺点是把tasks表所有的字段信息都取出来了，很多信息是
没有用的。

我们来看看更好的解决方案:

ruby script/generate migration add_tasks_count

我们新建一个migration，给projects表添加一个叫tasks_count的列:

class AddTasksCount < ActiveRecord::Migration
  def self.up
    add_column :projects, :tasks_count, :integer, :default => 0

    Project.reset_column_information
    Project.find(:all).each do |p|
      p.update_attribute :tasks_count, p.tasks.length
    end
  end

  def self.down
    remove_column :projects, :tasks_count
  end
end

我们还需要告诉Task类开启counter cache:

class Task < ActiveRecord::Base
  belongs_to :projects, :counter_cache => true
end

好了，我们把ProjectsController的index方法改回lazy loading，刷新页面，再看看log:

Project Lood (0.000295)  SELECT * FROM projects

可以看出，现在数据库只查询projects表的所有字段，查询时间也比前面的方案都短，性能大大提升。