用法1
Subject.where("position=?","").order("name")
用法2
与find方法不同的是，where方法返回的结果不是数组而是ActiveRelation，这意味着我们可以基于当前的查询结果继续设置条件进行查询。
Subject.where(:position =>"2").class
=> ActiveRecord::Relation
用法3
通过to_sql方法我们能看到Rails将我们的条件转换成的SQL语句以便于调试
Subject.where(:position =>"2").to_sql
=> "SELECT `subjects`.* FROM `subjects` WHERE `subjects`.`position` = '2'"
用法4
比如第一步先检索出position是2的所有对象，然后再根据name降序排列等等
Subject.where("position=?","").order("name desc")
=> [#<Subject id: 2, created_at: "2012-10-20 06:25:27", updated_at: "2012-10-20 15:10:36", name: "Second Subject", posit
ion: "">, #<Subject id: 4, created_at: "2012-10-20 15:14:07", updated_at: "2012-10-20 15:17:46", name: "Fourth Subject"
, position: "">]