python - SQLAlchemy - subquery in a WHERE clause

Question

I've just recently started using SQLAlchemy and am still having trouble wrapping my head around some of the concepts.

Boiled down to the essential elements, I have two tables like this (this is through Flask-SQLAlchemy):

class User(db.Model):
    __tablename__ = 'users'
    user_id = db.Column(db.Integer, primary_key=True)

class Posts(db.Model):
    __tablename__ = 'posts'
    post_id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('users.user_id'))
    post_time = db.Column(db.DateTime)

    user = db.relationship('User', backref='posts')

How would I go about querying for a list of users and their newest post (excluding users with no posts). If I was using SQL, I would do:

SELECT [whatever]
FROM posts AS p
    LEFT JOIN users AS u ON u.user_id = p.user_id
WHERE p.post_time = (SELECT MAX(post_time) FROM posts WHERE user_id = u.user_id)

So I know exactly the "desired" SQL to get the effect I want, but no idea how to express it "properly" in SQLAlchemy.

Edit: in case it's important, I'm on SQLAlchemy 0.6.6.

Answer 1

This should work (different SQL, same result):

t = Session.query(
    Posts.user_id,
    func.max(Posts.post_time).label('max_post_time'),
).group_by(Posts.user_id).subquery('t')

query = Session.query(User, Posts).filter(and_(
    User.user_id == Posts.user_id,
    User.user_id == t.c.user_id,
    Posts.post_time == t.c.max_post_time,
))

for user, post in query:
    print user.user_id, post.post_id

Where c stands for 'columns'