class Report(models.Model):
....
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
usage_id = models.PositiveIntegerField(verbose_name='ID', db_column='id')
usage = GenericForeignKey('content_type', 'usage_id')
....
Models. objects.all().values_list('usage', flat=True)
только работающий с GenericForeignKey ?