Ordering by subquery in SQLAlchemy

Question

I'm trying to select the newest threads (Thread) ordered descending by the time of the most recent reply to them (the reply is a Post model, that's a standard forum query). In SQL I'd write it like this:

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

How do I do such thing in SQLAlchemy? I tried something like this:

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

But it seems that I don't understand how scalars work. Reading docs for the 5th time won't help. Could someone help me write such query in SQLAlchemy? I use flask-sqlalchemy and MySQL for this app.

Answer 1

looks fine to me, here's a test:

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

output (note we're just printing the SQL here):

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

looks pretty much like your query