def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'total_examples') and self.total_examples is not None:
            _dict['total_examples'] = self.total_examples
        if hasattr(self, 'available') and self.available is not None:
            _dict['available'] = self.available
        if hasattr(self, 'processing') and self.processing is not None:
            _dict['processing'] = self.processing
        if hasattr(self, 'minimum_queries_added'
                  ) and self.minimum_queries_added is not None:
            _dict['minimum_queries_added'] = self.minimum_queries_added
        if hasattr(self, 'minimum_examples_added'
                  ) and self.minimum_examples_added is not None:
            _dict['minimum_examples_added'] = self.minimum_examples_added
        if hasattr(self, 'sufficient_label_diversity'
                  ) and self.sufficient_label_diversity is not None:
            _dict[
                'sufficient_label_diversity'] = self.sufficient_label_diversity
        if hasattr(self, 'notices') and self.notices is not None:
            _dict['notices'] = self.notices
        if hasattr(self, 'successfully_trained'
                  ) and self.successfully_trained is not None:
            _dict['successfully_trained'] = datetime_to_string(
                self.successfully_trained)
        if hasattr(self, 'data_updated') and self.data_updated is not None:
            _dict['data_updated'] = datetime_to_string(self.data_updated)
        return _dict