def one(self, filter_by=None):
<EOL>qs = self if filter_by is None else self.select(filter_by=filter_by)<EOL>if len(qs) == <NUM_LIT:0>:<EOL><INDENT>raise RecordDoesNotExistError("<STR_LIT>")<EOL><DEDENT>if len(qs) > <NUM_LIT:1>:<EOL><INDENT>raise MultipleRecordsReturnedError("<STR_LIT>")<EOL><DEDENT>return qs[<NUM_LIT:0>]<EOL>
Parameters
----------
filter_by: callable, default None
    Callable must take one argument (a record of table), and return True to keep record, or False to skip it.
    Example : .one(lambda x: x.name == "my_name").
    If None, records are not filtered.

Returns
-------
Record instance if one and only one record is found. Else raises.

Raises
------
RecordDoesNotExistError if no record is found
MultipleRecordsReturnedError if multiple records are found