SQLAlchemy删除关联对象

Question

我试图通过过滤其中一个关系中的列来批量删除关联表中的对象.我在SQLAlchemy中使用以下调用来进行删除

db.session.query(UserPaper).join(Paper, (UserPaper.paper_id ==
Paper.id)).filter(UserPaper.user_id == user.id).filter(Paper.journal_id 
== journal.id).delete()

并导致以下错误

OperationalError: (OperationalError) (1054, "Unknown column 'papers.journal_id' 
in 'where clause'") 'DELETE FROM userpapers WHERE userpapers.user_id = %s AND
papers.journal_id = %s' (1L, 1L)

如果没有最后的删除,SQLAlchemy查询就是

SELECT userpapers.user_id AS userpapers_user_id, userpapers.paper_id AS 
userpapers_paper_id, userpapers.created AS userpapers_created, 
userpapers.read_at AS userpapers_read_at, userpapers.score AS userpapers_score
FROM userpapers JOIN papers ON userpapers.paper_id = papers.id
WHERE userpapers.user_id = :user_id_1 AND papers.journal_id = :journal_id_1

哪个是对的.从错误中我可以看到,当我附加delete()到查询时,SQL语句的连接部分会丢失,并且数据库显然不知道如何查找papers.journal_id列.我不明白为什么会发生这种情况？

这是我的ORM对象的设置

class User(db.Model):
    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(255), unique=True)
    papers = db.relationship("UserPaper", backref=db.backref('users'), lazy='dynamic')

class Paper(db.Model):
    __tablename__ = 'papers'

    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(1024))
    journal_id = db.Column(db.Integer, db.ForeignKey('journals.id'))

class UserPaper(db.Model):
    __tablename__ = 'userpapers'

    user_id = db.Column(db.Integer, db.ForeignKey('users.id'), primary_key=True)
    paper_id = db.Column(db.Integer, db.ForeignKey('papers.id'), primary_key=True)
    paper = db.relationship("Paper", backref=db.backref('user_paper'))
    read_at = db.Column(db.DateTime)
    score = db.Column(db.Integer)

class Journal(db.Model):
    __tablename__ = 'journals'

    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(100), index = True, unique = True)
    papers = db.relationship('Paper', backref = 'journal', lazy = 'dynamic')

Answer 1

我在使用 MySQL 5.6 的 SQLALchemy 0.9 中遇到了同样的问题。它看起来像一个错误/限制。然而，一种更好的解决方法（与创建查询、循环结果并一一删除它们相比）是在两个后续查询中执行此任务：

    paperQuery = db.session.query(Paper.id)\
                                    filter(Paper.journal_id == journal.id)


    baseQuery  = db.session.query(UserPaper)\
                                    .filter(UserPaper.paper_id.in_(paperQuery.subquery()))
                                    .filter(UserPaper.user_id == user.id).delete(synchronize_session='fetch')

它对我来说效果很好，它也应该解决你的问题。