"""MySQL connector."""

from contextlib import contextmanager

from sqlalchemy import create_engine
from sqlalchemy.exc import SQLAlchemyError
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy.pool import NullPool

from contracts import config

if config.ENVIRONMENT == config.TEST_ENVIRONMENT:
    # For the test environment the database used is an in-memory sqlite db.
    # This db gets dropped immediately after the execution of the tests.
    _db_engine = create_engine('sqlite://')
else:
    _db_engine = create_engine(config.DB_URL, poolclass=NullPool)


BaseModel = declarative_base()


# A session() instance establishes all conversations with the database
# and represents a "staging zone" for all the objects loaded into the
# database session object. Any change made against the objects in the
# session won't be persisted into the database until you call
# session.commit(). If you're not happy about the changes, you can
# revert all of them back to the last commit by calling
# session.rollback()
#
# Description taken from:
# pythoncentral.io/introductory-tutorial-python-sqlalchemy/
_db_session = sessionmaker(bind=_db_engine)


@contextmanager
def db_session(expire_on_commit=True):
    """Provide transactional scope.

    Taken from http://docs.sqlalchemy.org/en/latest/orm/session_basics.html.
    Handles commit, rollback, and closing of sessions. Usage:

    with session_scope() as session:
        session.execute(query)

    """
    session = _db_session(expire_on_commit=expire_on_commit)
    try:
        yield session
        session.commit()
    except SQLAlchemyError:
        session.rollback()
        raise
    finally:
        session.close()