如何使用 SQLAlchemy 删除最后一行？

Question

我需要执行下一个查询：

DELETE FROM <table_name>
ORDER BY <column_name> DESC LIMIT 1

我正在尝试：

query = session.query(MyTable)
query.order_by(MyTable.my_column.desc()).limit(1).delete()

error: Can't call Query.update() or Query.delete() when order_by() has been called

我现在的方式（我想去掉循环）：

query = session.query(MyTable)
for item in query.order_by(MyTable.my_column.desc()).limit(1).all():
    query.filter(MyTable.id == item.id).delete()

Answer 1

首先获取最后一行

使用MySQL

SELECT * FROM Table ORDER BY ID DESC LIMIT 1

使用SQL

SELECT TOP 1 * FROM Table ORDER BY ID DESC

然后从第一个查询获取 ID 或这一行和运行这个查询

DELETE FROM table_name WHERE id=the_id_that_you_get_from_first_query

============ 更新 ===========

让我们从获取最后一行开始：

obj = session.query(ObjectRes).order_by(ObjectRes.id.desc()).first()

删除

Obj.query.filter(table.id == obj['id']).delete()

Answer 2

由于您正在处理 ORM 对象，因此您可以只检索“最后一个”项目，然后将其删除：

# ORM class for demo
class Team(Base):
    __tablename__ = "team"
    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    rank = Column(Integer)

    def __repr__(self):
        return f"<Team(id={self.id}, name='{self.name}', rank={self.rank})>"


Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)

# test data
with Session(engine) as session:
    session.add_all(
        [
            Team(name="Oilers", rank=3),
            Team(name="Flames", rank=1),
            Team(name="Canucks", rank=2),
        ]
    )
    session.commit()

# demo code
with Session(engine) as session:
    last_place_team = session.execute(
        select(Team).order_by(Team.rank.desc()).limit(1)
    ).scalar()
    print(last_place_team)  # <Team(id=1, name='Oilers', rank=3)>
    session.delete(last_place_team)
    session.commit()
    """SQL code:
    2021-04-25 14:08:16,251 INFO sqlalchemy.engine.Engine DELETE FROM team WHERE team.id = ?
    2021-04-25 14:08:16,252 INFO sqlalchemy.engine.Engine [generated in 0.00073s] (1,)
    """

如何使用 SQLAlchemy 删除最后一行？

How to delete last row using SQLAlchemy?

python

sql

orm

sqlalchemy