ruby-on-rails – PostgreSQL虚拟列列不存在？

SELECT (field_a + field_b + field_c) AS virtual_field,*
FROM "entities" 
WHERE ("entities".thing_id = 9999 AND (virtual_field > 0)) 
AND (boolean_field = 't') 
ORDER BY virtual_field DESC

不幸的是,我一直收到以下错误：

PGError: ERROR:  column "virtual_field" does not exist
LINE 1: ...ies" ("entities".thing_id = 9999 AND (virtual_fiel...
                                                 ^

错误信息非常明显,但如果我能找出正在尝试做的正确语法,我会被诅咒. field_a,field_b和field_c都是我的实体表中的实数列.

作为参考,我使用Rails(2.3.11)来编写查询.这是我正在使用的(匿名)代码：

Thing.entities.boolean_scope.find(:all,:select     => "(field_a + field_b + field_c) AS virtual_field,*",:conditions => ['virtual_field > ?',value],:order      => 'virtual_field DESC'
)

我的大脑让我失望了 – 任何人都可以帮助我搞清楚吗？