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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_model_at_related_field(model, attr):
 """ Looks up ``attr`` as a field of ``model`` and returns the related model class. If ``attr`` is not a relationship field, ``ValueError`` is raised. """


    field = model._meta.get_field(attr)

    if hasattr(field, 'related_model'):
        return field.related_model

    raise ValueError("{model}.{attr} ({klass}) is not a relationship field.".format(**{
        'model': model.__name__,
        'attr': attr,
        'klass': field.__class__.__name__,
    }))