Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'value') and self.value is not None:
            _dict['value'] = self.value
        if hasattr(self, 'metadata') and self.metadata is not None:
            _dict['metadata'] = self.metadata
        if hasattr(self, 'value_type') and self.value_type is not None:
            _dict['type'] = self.value_type
        if hasattr(self, 'synonyms') and self.synonyms is not None:
            _dict['synonyms'] = self.synonyms
        if hasattr(self, 'patterns') and self.patterns is not None:
            _dict['patterns'] = self.patterns
        if hasattr(self, 'created') and self.created is not None:
            _dict['created'] = datetime_to_string(self.created)
        if hasattr(self, 'updated') and self.updated is not None:
            _dict['updated'] = datetime_to_string(self.updated)
        return _dict