SQLAlchemy - WHERE 子句中的子查询 [英] SQLAlchemy - subquery in a WHERE clause

问题描述

我最近才开始使用 SQLAlchemy，但仍然无法理解一些概念.

归结为基本元素，我有两个这样的表(这是通过 Flask-SQLAlchemy):

class User(db.Model):__表名__ = '用户'user_id = db.Column(db.Integer, primary_key=True)类帖子(db.Model):__tablename__ = '帖子'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('用户', backref='posts')

我将如何查询用户列表及其最新帖子(不包括没有帖子的用户).如果我使用 SQL，我会这样做:

SELECT [随便]FROM 帖子 AS p左加入用户作为 u ON u.user_id = p.user_idWHERE p.post_time = (SELECT MAX(post_time) FROM posts WHERE user_id = u.user_id)

所以我确切地知道想要的"SQL 以获得我想要的效果，但不知道如何在 SQLAlchemy 中正确地"表达它.

以防万一，我使用的是 SQLAlchemy 0.6.6.

解决方案

这应该可以工作(不同的 SQL，相同的结果):

t = Session.query(Posts.user_id,func.max(Posts.post_time).label('max_post_time'),).group_by(Posts.user_id).subquery('t')查询 = 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，))对于用户，在查询中发布:打印 user.user_id, post.post_id

<块引用>

其中 c 代表列"

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.

解决方案

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'

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