I have two models, DocumentData & RecurringInvoice, which have a one to one relationship with each other. When I create an entry in the RecurringInvoice table, I want to pass in the DocumentData instance, instead of the id. But I get an error saying:

{'document_data': [ErrorDetail(string='Incorrect type. Expected pk value, received DocumentData.', code='incorrect_type')]} 

It works if I pass the id of the instance though. From what I know, we can pass in either the instance or the id to link the entries. So why does it fail when I pass the instance?

Relevant controller code:

try:
            # get the document data instance
            doc_data = DocumentData.objects.get(id=docDataID)
        except DocumentData.DoesNotExist:
            return Response({
                "message": "Document data does not exist"
            }, status=status.HTTP_400_BAD_REQUEST)

        invoice_serializer = RecurringInvoiceSerializer(
            data={
                "document_data": doc_data,
                "send_date": request.data["send_date"],
                "recipients": request.data["recipients"],
                "invoice_name": request.data["invoice_name"]
            }
        )

        if not invoice_serializer.is_valid():
            print(invoice_serializer.errors, invoice_serializer._errors)
            return Response({"message": "Failed to save invoice data"}, status=status.HTTP_400_BAD_REQUEST)

        invoice_serializer.save()

RecurringInvoice model:

class RecurringInvoice(models.Model):
    document_data = models.OneToOneField(
        DocumentData, on_delete=models.CASCADE, null=True, blank=True)
    send_date = models.IntegerField(
        default=1,
        validators=[MinValueValidator(1), MaxValueValidator(28)]
    )
    invoice_name = models.CharField(max_length=255)
    recipients = ArrayField(models.CharField(max_length=100))
    is_paused = models.BooleanField(default=False)
    stop_date = models.DateField(blank=True, null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    def __str__(self) -> str:
        return f"ID: {self.id}, Name: {self.invoice_name}, Active Till: {self.stop_date}"

    class Meta:
        verbose_name = "Recurring Invoice"
        verbose_name_plural = "Recurring Invoices"

DocumentData model:

class DocumentData(models.Model):
    invoices_sent_count = models.IntegerField(default=0)
    service_category = models.CharField(max_length=255, blank=True, null=True)
    from_company_name = models.CharField(max_length=255)
    from_company_number = models.CharField(
        max_length=255, blank=True, null=True
    )
    from_company_address = models.CharField(
        max_length=255, blank=True, null=True
    )