from sqlalchemy.orm import sessionmaker

from twisted.internet import threads

from twisted.python.threadpool import ThreadPool

POOL_SIZE=5     #how many threads should the db connector pool should have

class Data:

    def __init__(self):

        self.pool =  ThreadPool(minthreads=1, maxthreads=POOL_SIZE, name='database')

        self.reactor = None

d = Data()

def startPool(reactor): 

    d.pool.start()

    d.reactor = reactor

    d.reactor.addSystemEventTrigger('before', 'shutdown', d.pool.stop)

def run_in_db_thread(f):

    """Decorator to run DB queries in Twisted's thread pool"""

    def wrapper(*args, **kwargs):

        return threads.deferToThreadPool(d.reactor, d.pool, f, *args, **kwargs)

    return wrapper

class WithSession(object):

    '''a with statement for a database session connection'''

    def __init__(self, engine, autocommit=False):

        self.engine = engine

        self.autocommit=autocommit

    def __enter__(self):

        self.session = sessionmaker(bind=self.engine)()

        return self.session

    def __exit__(self,exc_type, exc_value, traceback):

        if exc_type is None:

            if self.autocommit:

                self.session.commit()

        else:

            self.session.rollback()

        self.session.close()

class DBDefer(object):

    '''a twisted sqlalchemy connector this Decorator adds a session parameter, with a valid session connection'''

    def __init__(self, engine, autocommit=False):

        self.autocommit=autocommit

        self.engine = engine

    def __call__(self, func):

        @run_in_db_thread

        def wrapper(*args, **kwargs):

            with WithSession(self.engine, self.autocommit) as session:

                return func(*args, session=session, **kwargs)

        return wrapper