SQLAlchemy - mapping one class to two tables

Question

I have two implementations of database queue (they use different tables) and want them to use objects of the same class. So, they both look really similar:

class AbstractDBQueue(object):
    def __init__(self, tablename):
        self.tablename = tablename
        self.metadata = MetaData()
        self.engine = create_engine('mysql+mysqldb://%s:%s@%s:%d/%s' % (
            settings.DATABASE.get('USER'),
            settings.DATABASE.get('PASSWORD'),
            settings.DATABASE.get('HOST') or '127.0.0.1',
            settings.DATABASE.get('PORT') or 3306,
            settings.DATABASE.get('NAME')
        ), encoding='cp1251', echo=True, pool_recycle=7200)
        self.metadata.bind = self.engine
        self.session = sessionmaker(bind=self.engine)()

    def setup_table(self, table, entity_name):
        self.table = table
        newcls = type(entity_name, (SMSMessage, ), {})
        mapper(newcls, table)
        return newcls

    def put(self, message=None, many_messages=[]):
        if message:
            self.session.add(message)
        else:
            for m in many_messages:
                self.session.add(m)
        self.session.commit()

    def get(self, limit=None):
        if limit:
            q = self.session.query(self.SMSClass).limit(limit)
        else:
            q = self.session.query(self.SMSClass)
        smslist = []
        for sms in q:
            smslist.append(sms)
        self.session.expunge_all()
        return smslist

class DBQueue(AbstractDBQueue):
    """
    MySQL database driver with queue interface
    """
    def __init__(self):
        self.tablename = settings.DATABASE.get('QUEUE_TABLE')
        super(DBQueue, self).__init__(self.tablename)
        self.logger = logging.getLogger('DBQueue')
        self.SMSClass = self.setup_table(Table(self.tablename, self.metadata, autoload=True), "SMSQueue")

class DBWorkerQueue(AbstractDBQueue):
    """
    MySQL database driver with queue interface for separate workers queue
    """

    def __init__(self):
        self.tablename = settings.DATABASE.get('WORKER_TABLE')
        super(DBWorkerQueue, self).__init__(self.tablename)
        self.logger = logging.getLogger('DBQueue')
        self.SMSClass = self.setup_table(Table(self.tablename, self.metadata, autoload=True), "SMSWorkerQueue")

    def _install(self):
        self.metadata.create_all(self.engine)

SMSMessage is the name of the class I want to use. The map_class_to_table() function is a hack I've found in SQLAlchemy documentation: http://www.sqlalchemy.org/trac/wiki/UsageRecipes/EntityName

But it doesn't seems to help - when the first queue instance maps SMSMessage to it's table, then all objects I pass to second queue's put() are implicitly casted to first queue's mapped class, and second database is still empty after session.commit() .

I need to use both queues at the same time, maybe even using threads (I think, pool connection will be useful), but I just can't make this work. Could you help, please?

Answer 1

I think your problem relates to the tablename variable. It's a class variable which gets defined when you create the class, and then does not change . So, it will be the same for both of your instances, when they access it with self.tablename . To fix this, move it inside the init function, and make it a self.tablename . That will initialize it each time you create a new object.