# third-party imports
from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

Model = declarative_base(name='Model')


class SessionBase(Model):
    """The base object for all SQLAlchemy enabled models. All other models must extend this.
    """
    __abstract__ = True
 
    id = Column(Integer, primary_key=True)

    engine = None
    """Database connection engine of the running aplication"""
    sessionmaker = None
    """Factory object responsible for creating sessions from the  connection pool"""
    transactional = True
    """Whether the database backend supports query transactions. Should be explicitly set by initialization code"""
    poolable = True
    """Whether the database backend supports query transactions. Should be explicitly set by initialization code"""


    @staticmethod
    def create_session():
        """Creates a new database session.
        """
        return SessionBase.sessionmaker()


    @staticmethod
    def _set_engine(engine):
        """Sets the database engine static property
        """
        SessionBase.engine = engine
        SessionBase.sessionmaker = sessionmaker(bind=SessionBase.engine)


    @staticmethod
    def connect(dsn, debug=False):
        """Create new database connection engine and connect to database backend.

        :param dsn: DSN string defining connection.
        :type dsn: str
        """
        e = None
        if SessionBase.poolable:
            e = create_engine(
                    dsn,
                    max_overflow=50,
                    pool_pre_ping=True,
                    pool_size=20,
                    pool_recycle=10,
                    echo=debug,
                )
        else:
            e = create_engine(
                    dsn,
                    echo=debug,
                    )

        SessionBase._set_engine(e)


    @staticmethod
    def disconnect():
        """Disconnect from database and free resources.
        """
        SessionBase.engine.dispose()
        SessionBase.engine = None