code: def get(cls, expression):
          
          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)
docs: 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.