def get_related(self, instance, number):
        """
        Implement high level cache system for get_related.
        """
        cache = self.cache
        cache_key = '%s:%s' % (instance.pk, number)
        if cache_key not in cache:
            related_objects = super(CachedModelVectorBuilder,
                                    self).get_related(instance, number)
            cache[cache_key] = related_objects
            self.cache = cache
        return cache[cache_key]
Implement high level cache system for get_related.