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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 get(cls, expression):
 """ Retrieve the model instance matching the given expression. If the number of matching results is not equal to one, then a ``ValueError`` will be raised. :param expression: A boolean expression to filter by. :returns: The matching :py:class:`Model` instance. :raises: ``ValueError`` if result set size is not 1. """

        executor = Executor(cls.__database__)
        result = executor.execute(expression)
        if len(result) != 1:
            raise ValueError('Got %s results, expected 1.' % len(result))
        return cls.load(result._first_or_any(), convert_key=False)