看这个页面代码:

<% for task in @tasks %>
  <%= link_to task.name, task_path(task) %> in <%= task.project.name %>
<% end %>

上面的代码对每个task对象，取得project的name属性并显示，看看log:

Project Lood (0.000131)  SELECT * FROM projects WHERE (projects.id = 330)
Project Lood (0.000156)  SELECT * FROM projects WHERE (projects.id = 330)
Project Lood (0.000136)  SELECT * FROM projects WHERE (projects.id = 330)
Project Lood (0.000138)  SELECT * FROM projects WHERE (projects.id = 330)
Project Lood (0.000133)  SELECT * FROM projects WHERE (projects.id = 330)
Project Lood (0.000128)  SELECT * FROM projects WHERE (projects.id = 330)
...
Completed in 0.14792 (6 reqs/sec) | ...

由于ActiveRecord默认是延迟加载的，这样对每个task对象去取project对象时又会查询一次数据库
对于这种情况，我们使用eager loading就比较合适了:

class TasksController < ApplicationController
  def index
    @tasks = Task.find(:all, :include => :project)
  end
end

:include指定我们将project属性一起查询出来，我们再看看log:

Task Lood Including Associations (0.000681)  SELECT tasks.'id' AS t0_r0, tasks.'name' AS t0_r1, tasks.'project_id' AS
t0_r2, projects.'id' AS t1_r0, projects.'name' AS t1_r1 FROM tasks LEFT OUTER JOIN projects ON projects.id = tasks.pro
ject_id
...
Completed in 0.03464 (28 reqs/sec) | ...