Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'trait_id') and self.trait_id is not None:
            _dict['trait_id'] = self.trait_id
        if hasattr(self, 'name') and self.name is not None:
            _dict['name'] = self.name
        if hasattr(self, 'category') and self.category is not None:
            _dict['category'] = self.category
        if hasattr(self, 'percentage') and self.percentage is not None:
            _dict['percentage'] = self.percentage
        return _dict