如何使用Join和Aggregate編寫SQLAlchemy查詢？

Question

我有一個表有3列：類型，內容和時間（一個整數）。對於每個'類型'，我想選擇具有最大（最近）'時間'整數和相應數據的條目。我怎樣才能使用SQLAlchemy和Python來做到這一點？我可以使用SQL執行以下操作：

select 
    c.type, 
    c.time, 
    b.data 
from 
    parts as b 

inner join 

    (select 
    a.type, 
    max(a.time) as time 
    from parts as a 
    group by a.type) as c 

on 

b.type = c.type and 
b.time = c.time 

但是，如何在SQLAlchemy中完成此操作？

表映射：

class Structure(Base): 
    __tablename__ = 'structure' 
    id = Column(Integer, primary_key=True) 
    type = Column(Text) 
    content = Column(Text) 
    time = Column(Integer) 

    def __init__(self, type, content): 
     self.type = type 
     self.content = content 
     self.time = time.time() 

    def serialise(self): 
     return {"type" : self.type, 
      "content" : self.content}; 

試圖進行查詢：

max = func.max(Structure.time).alias("time") 
    c = DBSession.query(max)\ 
     .add_columns(Structure.type, Structure.time)\ 
     .group_by(Structure.type)\ 
     .subquery() 
    c.alias("c") 

    b = DBSession.query(Structure.content)\ 
     .add_columns(c.c.type, c.c.time)\ 
     .join(c, Structure.type == c.c.type) 

給我：

sqlalchemy.exc.OperationalError: (OperationalError) near "(": syntax error u'SELECT structure.content AS structure_content, anon_1.type AS anon_1_type, anon_1.t ime AS anon_1_time \nFROM structure JOIN (SELECT time.max_1 AS max_1, structure.type AS type, structure.time AS time \nFROM max(structure.time) AS time, structu re GROUP BY structure.type) AS anon_1 ON structure.type = anon_1.type'()

我基本上是在黑暗中刺傷，所以任何幫助將是讚賞。

Answer 1

嘗試以下使用子查詢代碼：

subq = (session.query(
      Structure.type, 
      func.max(Structure.time).label("max_time") 
     ). 
     group_by(Structure.type) 
     ).subquery() 

qry = (session.query(Structure). 
     join(subq, and_(Structure.type == subq.c.type, Structure.time == subq.c.max_time)) 
     ) 

print qry 

生產SQL：

SELECT structure.id AS structure_id, structure.type AS structure_type, structure.content AS structure_content, structure.time AS structure_time 
FROM structure 
JOIN (SELECT structure.type AS type, max(structure.time) AS max_time 
     FROM structure GROUP BY structure.type) AS anon_1 
    ON structure.type = anon_1.type 
    AND structure.time = anon_1.max_time