"""Connection to DB."""
from contextlib import contextmanager

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

from images import config


# please don't use the following private variables directly;
# use db_session
_db_engine = create_engine(
    config.AR_CONNECTION_URL, poolclass=config.POOL_CLASS)
_db_session = sessionmaker(bind=_db_engine)


@contextmanager
def db_session():
    """Provide a transactional scope around a series of operations.

    Taken from http://docs.sqlalchemy.org/en/latest/orm/session_basics.html.
    This handles rollback and closing of session, so there is no need
    to do that throughout the code.

    Usage:
        with db_session() as session:
            session.execute(query)
    """
    session = _db_session()
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise
    finally:
        session.close()