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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 serialize(self, value, entity, request):
 """ Propagate to nested fields. :returns: data dictionary or ``None`` if no fields are present. """


        self._validate_existence(value)
        self._run_validators(value)

        if not value:
            return value

        return self.factory.serialize(value, request)