Rails counter cache

这次就是讲用_count字段来缓存has_many的计数 

看Project和Task的例子:

Projects

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