Return a json dictionary representing this model.

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