Question

I want to get a row from mysql database, then modiy some fields and commit. Before I commit, updating this row shoud be forbidden. so I use with_lockmode(‘update’). I use the
following RoutingSession and I find it use Slave to do this query. But I want to do this query using Master, what should I do in get_bind???

class RoutingSession(Session):
    def get_bind(self, mapper=None, clause=None):
        if self._flushing:

            return engines['master']
        else:
            return engines['slave']

row = session.query(SomeTable).filter(SomeTable.id = 1).with_lockmode('update').one
row.somefield = 'newcontent'
session.commit()

Answer 1

I’m assuming you got this recipe from my blog post at Django-style Database Routers in SQLAlchemy. If you look in the section “manual access” you’ll see a recipe for when you need to explicitly point the routing session to a certain node:

class RoutingSession(Session):

    def get_bind(self, mapper=None, clause=None ):
        if self._name:
            return engines[self._name]
        elif mapper and issubclass(mapper.class_, OtherBase):
            return engines['other']
        elif self._flushing:
            return engines['master']
        else:
            return engines[
                    random.choice(['slave1','slave2'])
                ]

    _name = None
    def using_bind(self, name):
        s = RoutingSession()
        vars(s).update(vars(self))
        s._name = name
        return s

Then you just do your operation with that bind explicitly:

m1 = Session().using_bind("master").query(Model1).first()