Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """
    alembic_config = config.get_section(config.config_ini_section)
    alembic_config['sqlalchemy.url'] = os.environ['DATABASE_URI']

    connectable = engine_from_config(
        alembic_config,
        prefix='sqlalchemy.',
        poolclass=pool.NullPool)

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata
        )

        with context.begin_transaction():
            context.run_migrations()