Issue with adding rows with generated field about django-import-export HOT 6 OPEN

class OperationsExportShipments(models.Model):
    shipment_id = models.CharField(primary_key=True)
    shipment_status = models.ForeignKey(OperationsExportShipmentsStatuses, models.DO_NOTHING, db_column='shipment_status')
    checked_in = models.BooleanField()
    start_date = models.DateField(blank=True, null=True)
    end_date = models.DateField(blank=True, null=True)
    fba_shipment = models.OneToOneField(FbaShipments, models.DO_NOTHING, db_column='fba_shipment',null=True,blank=True)
    walmart_shipment = models.OneToOneField(WalmartShipments, models.DO_NOTHING, db_column='walmart_shipment',null=True,blank=True)
    shipment_carrier = models.ForeignKey(WarehouseCarriers, models.DO_NOTHING, db_column='shipment_carrier',null=True,blank=True)
    mock_generated_column =   models.GeneratedField(
        expression=RawSQL("1", []),
        output_field=models.IntegerField(),
        db_persist=True
    )

    
    class Meta:
        managed = True
        db_table = 'operations_export_shipments'
        
        #contraint that exactly one of fba_shipment or walmart_shipment must be set
        constraints = [
            models.CheckConstraint(
                check=(
                    (Q(fba_shipment__isnull=False) & Q(walmart_shipment__isnull=True)) |
                    (Q(fba_shipment__isnull=True) & Q(walmart_shipment__isnull=False))
                ),
                name='check_fba_walmart_shipment_exclusivity'
            ),
            #constraint that checked_in can only be True if shipment_status is 'ARRIVED'
            models.CheckConstraint(
                check=(
                    Q(checked_in=False) | 
                    (Q(checked_in=True) & Q(shipment_status='ARRIVED'))
                ),
                name='check_checked_in_with_status_arrived'
            )
        ]

class OperationsExportShipmentsResource(resources.ModelResource):
    class Meta:
        model = OperationsExportShipments
        fields = (
            "fba_shipment","walmart_shipment",
            "shipment_status",
            "checked_in"
        )

    def before_import_row(self, row, **kwargs):
        """
        Override this method to add logic before saving an instance.
        Use this to determine if a row corresponds to an FBA shipment or Walmart shipment and handle accordingly.
        """
        row["shipment_status"] = row["shipment_status"].upper().replace(" ","_")
        if not row["fba_shipment"]:
            row["fba_shipment"] = None
        if not row["walmart_shipment"]:
            row["walmart_shipment"] = None

        if row["checked_in"].lower().strip() == "yes":
            row["checked_in"] = True
        elif row["checked_in"].lower().strip() == "no":
            row["checked_in"] = False
        else:
            row["checked_in"] = None
        print(row)
        super().before_import_row(row, **kwargs)



    def get_instance(self, instance_loader, row):
        """
        Override to custom handle identifying existing instances.
        """
        # print(row)
        # self.Meta.model.objects.get(fba_shipment=row['fba_shipment'],
                                           # walmart_shipment=row['walmart_shipment'])
        print("stage 2")
        try:

            return self.Meta.model.objects.get(fba_shipment=row['fba_shipment'],
                                                walmart_shipment=row['walmart_shipment'])
        except self.Meta.model.DoesNotExist:
            print("no instance foudn!")
            return None