gistfile1.py

class SmartPostgresTarget(PostgresTarget):
    """PostgresTarget with possibility to specify an existence query"""
    def __init__(self, host, database, user, password,
                 table,
                 exists_query,
                 update_id=None):
        update_id = update_id or exists_query
        self.exists_query = exists_query
        super(SmartPostgresTarget, self).__init__(
            host, database, user, password,
            table, update_id)

    def exists(self, connection=None):
        """ Checks if data exists according to normal PostgresTarget cache

        If NOT, check it using a custom exists query and if this says data
        exists, touch() the target to refresh the cache """
        cache_complete = super(SmartPostgresTarget, self).complete(connection)
        if cache_complete:
            return True

        if connection is None:
            connection = self.connect()
            connection.autocommit = True
        cursor = connection.cursor()
        cursor.execute(self.exists_query)
        row = cursor.fetchone()
        cursor.close()
        exists = row is not None
        if exists:
            self.touch(connection)
        return exists