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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_dict(self):
 """Return a dict representation of the record."""

        d = {}
        if self.query_params_match is not None:
            d['@query_params_match'] = self.query_params_match
        if self.query_person_match is not None:
            d['@query_person_match'] = self.query_person_match
        if self.valid_since is not None:
            d['@valid_since'] = datetime_to_str(self.valid_since)
        if self.source is not None:
            d['source'] = self.source.to_dict()
        d.update(self.fields_to_dict())
        return d