def validate(
            self, schema_id, validation_function=None,
            validation_error_handler=None):
        """
        A decorator that is used to validate incoming requests data
        against a schema

            swagger = Swagger(app)

            @app.route('/pets', methods=['POST'])
            @swagger.validate('Pet')
            @swag_from("pet_post_endpoint.yml")
            def post():
                return db.insert(request.data)

        This annotation only works if the endpoint is already swagged,
        i.e. placing @swag_from above @validate or not declaring the
        swagger specifications in the method's docstring *won't work*

        Naturally, if you use @app.route annotation it still needs to
        be the outermost annotation

        :param schema_id: the id of the schema with which the data will
            be validated

        :param validation_function: custom validation function which
            takes the positional arguments: data to be validated at
            first and schema to validate against at second

        :param validation_error_handler: custom function to handle
            exceptions thrown when validating which takes the exception
            thrown as the first, the data being validated as the second
            and the schema being used to validate as the third argument
        """

        if validation_function is None:
            validation_function = self.validation_function

        if validation_error_handler is None:
            validation_error_handler = self.validation_error_handler

        def decorator(func):
            @wraps(func)
            def wrapper(*args, **kwargs):
                specs = get_schema_specs(schema_id, self)
                validate(
                    schema_id=schema_id, specs=specs,
                    validation_function=validation_function,
                    validation_error_handler=validation_error_handler)
                return func(*args, **kwargs)

            return wrapper

        return decorator