01_models.py

class MyManager(models.Manager):
    use_for_related_fields = True

    def get_query_set(self):
        return super(MyManager, self).get_query_set().filter(is_deleted=False)
      
        
class MyModel(gis_models.Model):
    is_deleted = models.BooleanField(default=False, db_index=True)
    
    objects = FlatManager()
    all_objects = models.Manager()
    
    
class MyModelProxy(MyModel):
    objects = models.Manager()

    class Meta:
        proxy = True

02_import_management_cmd.py

# i'm using sql alchemy here, to get objects from another database
for obj in self.session.query(ModelLegacy).filter(ModelLegacy.acc_id == acc.id):
    try:
        new_object = MyModelProxy.all_objects.get(created=obj.date_added)
    except MyModelProxy.DoesNotExist:
        new_object = MyModelProxy(created=obj.date_added)
    new_object.is_deleted = obj.deleted
    new_object.user = new_user
    new_object.save()

    # now here, the second time i run this script, that happens:
    django.db.utils.IntegrityError: duplicate key value violates unique constraint "myapp_mymodel_pkey"
    DETAIL:  Key (id)=(5453) already exists.

    # if i follow it with debugging:
    -> new_object.save()
    (Pdb) new_object
    <MyModel: asdf>
    (Pdb) new_object.pk
    5453
    (Pdb) new_object.save()
    *** DoesNotExist: Flat matching query does not exist.

    # if i follow this further:
    https://github.com/django/django/blob/master/django/db/models/base.py#L715
    (Pdb) cls._base_manager.count()
    608
    (Pdb) cls.all_objects.count()
    632

    # so i think, djano uses 'objects' manager as _base_manager in the background
    # and can't find my is_deleted model