Filtering out results in Automatic Managers

Hi,

In https://docs.djangoproject.com/en/dev/topics/db/managers/#do-not-filter-away-any-results-in-this-type-of-manager-subclass, it is written that

"A manager that filters results in get_queryset() is not appropriate for use as an automatic manager."

Can you please tell me why is it not appropriate because this is exactly what I want, filter results in backward RelatedManagers?

It is written, "If you override the get_queryset() method and filter out any rows, Django will return incorrect results.". What type of incorrect results will Django return?

Thank you

Example:

class TrashableManager(models.Manager):

    use_for_related_fields = True

    def get_query_set(self):

        return super(TrashableManager, self).get_query_set().filter(trashed=False)

class TrashedManager(models.Manager):

    def get_query_set(self):

        return super(TrashableManager, self).get_query_set().filter(trashed=True)

class Trashable(models.Model):

    trashed = models.BooleanField(default=False)

    datetime_trashed = models.DateTimeField(blank=True, null=True)

    objects = TrashableManager()

    trashed_objects = TrashedManager()

    def trash(self):

        if not self.trashed:

            self.datetime_trashed = datetime.now()

        self.trashed = True

        self.save()

    class Meta:

        abstract = True

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