'Takes a model object and returns results the backend thinks are similar.
This method MUST be implemented by each backend, as it will be highly
specific to each one.'

def more_like_this(self, model_instance, additional_query_string=None, result_class=None):

    raise NotImplementedError(u"Subclasses    must    provide    a    way    to    fetch    similar    record    via    the    'more_like_this'    method    if    supported    by    the    backend.")
