"""DB Connector.

Manages interactions with Relational Database (MySQL and SQLite).
"""

from contextlib import contextmanager

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

from backend import config


db_engine = create_engine(
       config.DB_URL, poolclass=config.POOL_CLASS)

# please don't use _db_session directly;
# instead use db_session
_db_session = sessionmaker(bind=db_engine)

BaseModel = declarative_base()


@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()