Question

我正在尝试选择最新回复时间顺序下降的最新线程（Thread）（回复是Post模型，这是标准的论坛查询）。在SQL中我会这样写：

SELECT * FROM thread AS t ORDER BY (SELECT MAX(posted_at) FROM post WHERE thread_id = t.id) DESC

我如何在SQLAlchemy中执行此类操作？我试过这样的事情：

scalar = db.select[func.max(Post.posted_at)].where(Post.thread_id == Thread.id).as_scalar()
threads = Thread.query.order_by(scalar.desc()).all()

但似乎我不明白标量是如何工作的。第五次阅读文档无济于事。有人可以帮我在SQLAlchemy中编写这样的查询吗？我在这个应用程序中使用flask-sqlalchemy和MySQL。

Answer 1

对我来说很好，这是一个测试：

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Thread(Base):
    __tablename__ = 'thread'

    id = Column(Integer, primary_key=True)


class Post(Base):
    __tablename__ = 'post'

    id = Column(Integer, primary_key=True)
    thread_id = Column(Integer, ForeignKey('thread.id'))
    posted_at = Column(String)

s = Session()

scalar = select([func.max(Post.posted_at)]).where(Post.thread_id == Thread.id).as_scalar()

q = s.query(Thread).order_by(scalar.desc())

print q

输出（注意我们只是在这里打印SQL）：

SELECT thread.id AS thread_id 
FROM thread ORDER BY (SELECT max(post.posted_at) AS max_1 
FROM post 
WHERE post.thread_id = thread.id) DESC

看起来非常像您的查询

在SQLAlchemy中按子查询排序

1 个答案: