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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))