Check value is a valid JSON string, raise ValidationError on
        error.

def validate(self, value, model_instance):
        """Check value is a valid JSON string, raise ValidationError on
        error."""
        if isinstance(value, six.string_types):
            super(JSONField, self).validate(value, model_instance)
            try:
                json.loads(value)
            except Exception as err:
                raise ValidationError(str(err))