Rails Activerecord Relation:使用子查询作为SQL select语句的表

Question

有人可以帮我弄清楚如何使用Rails编写以下SQL(我使用的是Rails 4)Activerecord方法？我知道你可以用find_by_sql做到这一点,但我想保留活动的记录关系.这是我正在尝试创建的postGreSQL数据库的sql:

SELECT * FROM 
(SELECT DISTINCT ON(table_a.id) table_a.name as alias_a, table_b.id, table_b.time
FROM table_1
LEFT OUTER JOIN table_b ON table_a.id = table_b.id
ORDER BY table_a.id, table_b.time asc) AS subquery
ORDER BY alias_a asc

对于我的子查询,我有以下内容(它生成上面子查询的sql):

@subquery = table_a.select("DISTINCT ON(table_a.id) table_a.name as alias_a, table_b.time")     
@subquery = @subquery.joins("LEFT OUTER JOIN table_b ON table_a.id = table_b.id")
@subquery = @subquery.order("table_a.id, table_b.time asc")

但是,我无法弄清楚如何编写一个使用@subquery作为外部select语句的表的select语句.

Answer 1

使用from()Active Record界面中的方法.

例如:

@subquery = table_a.select("DISTINCT ON(table_a.id) table_a.name as alias_a, table_b.time")     
@subquery = @subquery.joins("LEFT OUTER JOIN table_b ON table_a.id = table_b.id")
@subquery = @subquery.order("table_a.id, table_b.time asc")

然后在外部查询中使用它:

@query = OtherModel.from("(#{@subquery.to_sql}) table_name, other_model_table, etc ...").where(:field => table_name.alias_a) ...etc.