Reads a flow object from the database.

def ReadFlowObject(self, client_id, flow_id, cursor=None):
    """Reads a flow object from the database."""
    query = ("SELECT " + self.FLOW_DB_FIELDS +
             "FROM flows WHERE client_id=%s AND flow_id=%s")
    cursor.execute(
        query,
        [db_utils.ClientIDToInt(client_id),
         db_utils.FlowIDToInt(flow_id)])
    result = cursor.fetchall()
    if not result:
      raise db.UnknownFlowError(client_id, flow_id)
    row, = result
    return self._FlowObjectFromRow(row)