'Same as db.define_table except that it does not repeat
a table definition if the table is already defined.'

@classmethod
def define_table(cls, tablename, *fields, **args):

    db = current.db
    if hasattr(db, tablename):
        table = ogetattr(db, tablename)
    else:
        table = db.define_table(tablename, *fields, **args)
    return table
