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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 related_items_changed(self, instance, related_manager):
 """ Stores the number of comments. A custom ``count_filter`` queryset gets checked for, allowing managers to implement custom count logic. """

        try:
            count = related_manager.count_queryset()
        except AttributeError:
            count = related_manager.count()
        count_field_name = list(self.fields.keys())[0] % \
                           self.related_field_name
        setattr(instance, count_field_name, count)
        instance.save()