SQLAlchemy: How to Delete with join

Question

I have trouble doing such thing bottom with SQLAlchemy:

DELETE a FROM a INNER JOIN b ON b.`aId` = a.`Id` WHERE `b`.`xxx` = ?;

As the post here: SQLAlchemy: Create delete query using self-join on MySQL

I've got it's hard to do delete in SQLAlchemy with join.

So I'm now doing like this:

session.execute('DELETE a FROM a INNER JOIN b ON b.`aId` = a.`Id` WHERE `b`.`xxx` = %d;'%xxx)

But it just annoy me a lot like about: SQL Injection thing, etc..

Is there any way using SQLAlchemy to solve the problem here? Thanks!

Answer 1

SQLAlchemy 1.2 and up support multi table deletes for some dialects (at the time of writing Postgresql, MySQL and Microsoft SQL Server):

In [18]: a = table('a', column('x'))

In [19]: b = table('b', column('x'))

In [20]: c = table('c', column('x'), column('y'))

In [21]: a.delete().\
    ...:     where(a.c.x == b.c.x).\
    ...:     where(b.c.x == c.c.x).\
    ...:     where(c.c.y == 1)
Out[21]: <sqlalchemy.sql.dml.Delete object at 0x7f3577d89160>

In [22]: print(_.compile(dialect=mysql.dialect()))
DELETE FROM a USING a, b, c WHERE a.x = b.x AND b.x = c.x AND c.y = %s

and the same using a Session and Declarative:

In [2]: class Foo(Base):
   ...:     __tablename__ = 'foo'
   ...:     id = Column(Integer, primary_key=True)

In [3]: class Bar(Base):
   ...:     __tablename__ = 'bar'
   ...:     id = Column(Integer, primary_key=True)
   ...:     foo_id = Column(ForeignKey(Foo.id))
   ...:     

In [4]: class Baz(Base):
   ...:     __tablename__ = 'baz'
   ...:     id = Column(Integer, primary_key=True)
   ...:     bar_id = Column(ForeignKey(Bar.id))
   ...:     val = Column(Integer)
   ...:     

In [5]: session.query(Foo).\
   ...:     filter(Foo.id == Bar.foo_id,
   ...:            Bar.id == Baz.bar_id,
   ...:            Baz.val == 1).\
   ...:     delete(synchronize_session=False)
   ...:            

which would emit

DELETE FROM foo USING foo, bar, baz
WHERE foo.id = bar.foo_id AND bar.id = baz.bar_id AND baz.val = %(val_1)s

Answer 2

Here is the way I did it:

map_ids = session.query(Table1.map_id). \
    filter(Table1.xxxx == 'xxx_value')

q = session.query(Table2). \
    filter(Table2.map_id.in_(map_ids.subquery()))

q.delete(synchronize_session=False)

The key here are you create subquery by finding ids that has to be deleted. Then you use 'in_' operator to select records to be deleted. 'synchoronize_session=False' will execute query without updating session. This gives the best performance boost.

Answer 3

If you read the documentation for session.execute you will see that you should do something like this:

session.execute(
    'DELETE a FROM a INNER JOIN b ON b.`aId` = a.`Id` WHERE `b`.`xxx` = :param',
    {"param": 5}
)