6

鉴于以下声明：

p = db.query(Profile).options(joinedload('*')).filter_by(id=p.id).limit(1).one()

我将得到一个子查询 + 一个连接，而不是一个“纯”连接：

SELECT [...] 
FROM (SELECT profile.id AS profile_id, ...
FROM profile 
WHERE profile.id = %(id_1)s 
LIMIT %(param_1)s) AS anon_1 LEFT OUTER JOIN city AS city_1 ON city_1.id = anon_1.profile_city LEFT OUTER JOIN country AS country_1 ON country_1.id = city_1.country LEFT OUTER JOIN state AS state_1 ON country_1.id = state_1.country LEFT OUTER JOIN state AS state_2 ON state_2.id = city_1.state LEFT OUTER JOIN country AS country_2 ON country_2.id = state_2.country LEFT OUTER JOIN state AS state_3 ON state_3.id = city_1.state LEFT OUTER JOIN country AS country_3 ON country_3.id = state_3.country LEFT OUTER JOIN starred AS starred_1 ON anon_1.profile_id = starred_1.star LEFT OUTER JOIN profiletext AS profiletext_1 ON anon_1.profile_id = profiletext_1.profile LEFT OUTER JOIN starred AS starred_2 ON anon_1.profile_id = starred_2.idprofile LEFT OUTER JOIN photo AS photo_1 ON anon_1.profile_id = photo_1.profile LEFT OUTER JOIN gps AS gps_1 ON anon_1.profile_id = gps_1.idprofile

但我真正需要的是：

SELECT ...
FROM profile LEFT OUTER JOIN city AS city_1 ON city_1.id = profile.city LEFT OUTER JOIN country AS country_1 ON country_1.id = city_1.country LEFT OUTER JOIN state AS state_1 ON country_1.id = state_1.country LEFT OUTER JOIN state AS state_2 ON state_2.id = city_1.state     
LEFT OUTER JOIN country AS country_2 ON country_2.id = state_2.country LEFT OUTER JOIN state AS state_3 ON state_3.id = city_1.state LEFT OUTER JOIN country AS country_3 ON country_3.id = state_3.country LEFT OUTER JOIN starred AS starred_1 ON profile.id = starred_1.star LEFT OUTER JOIN profiletext AS profiletext_1 ON profile.id = profiletext_1.profile LEFT OUTER JOIN starred AS starred_2 ON profile.id = starred_2.idprofile LEFT OUTER JOIN photo AS photo_1 ON profile.id = photo_1.profile LEFT OUTER JOIN gps AS gps_1 
ON profile.id = gps_1.idprofile                                                                                                                                                                                                    
WHERE profile.id = 4 
limit 1;

即没有子查询。

数据库：PostgreSQL 9.2

4

1 回答 1

3

根据The Zen of Eager Loading，这似乎是预期的行为

使用联合预加载时，如果查询包含影响从外部返回到连接的行的修饰符，例如使用 DISTINCT、LIMIT、OFFSET 或等效项时，完成的语句首先包装在子查询中，并且连接专门用于加入的急切加载应用于子查询。SQLAlchemy 的加入渴望加载更进一步，然后再进一步十英里，以绝对确保它不会影响查询的最终结果，只影响集合和相关对象的加载方式，无论查询的格式是什么。

我意识到这是一个老问题，但是生成的 SQL 不起作用是有原因的吗？

于 2013-05-06T16:49:56.613 回答