Sqlalchemy: 联合加载 + 限制

Question

给定以下语句：

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

我会得到一个子查询加上一个连接，而不是一个“纯粹”的连接：

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;

也就是说，不能有子查询。

数据库：postgresql 9.2

Answer 1

这看起来是正常的行为，按照急切加载的原则来讲。

在使用连接急切加载时，如果查询中包含一些会影响返回行的修改器，比如使用 DISTINCT（去重）、LIMIT（限制返回数量）、OFFSET（偏移量）或类似的东西，最终生成的语句会先被包裹在一个子查询中，然后专门用于连接急切加载的连接会应用到这个子查询上。SQLAlchemy 的连接急切加载做得非常细致，确保它不会影响查询的最终结果，只是改变了集合和相关对象的加载方式，无论查询的格式是什么样的。

我知道这是个老问题，但生成的 SQL 为什么不工作呢？