Django: Check if an instance exists before creating - django

Question

I have two models Purchaser and paymentInvoice, I want to make sure i don't create a duplicate Purchaser object when i'm creating a new paymentInvoice for the same Purchaser individual/instance.
Basically i have a Purchaser by the name Becky, so when i want to create an invoice for Becky 1st i want to make sure if the name Becky exists in Purchaser if it does, create paymentInvoice object with Becky taking the field invoiceOwner. If Becky doesn't exist in Purchaser, create an instance of that purchaser in Purchaser then use that instance name to create paymentInvoice object.
Models file
class Purchaser(models.Model):
name = models.CharField(max_length=50)
phone = models.CharField(max_length=20)
email = models.EmailField(max_length=255, blank=True, null=True)
image = models.ImageField(default='default.png', upload_to='customer_photos/%Y/%m/%d/')
data_added = models.DateField(default=datetime.date.today)
def __str__(self):
return self.name
class paymentInvoice(models.Model):
invoiceNo = models.CharField(max_length=50, unique=True, default=increment_invoice_number)
invoiceOwner = models.ForeignKey(Purchaser, on_delete=models.CASCADE, related_name="invoice_detail")
product = models.CharField(max_length=50, blank=True)
date = models.DateField(default=datetime.date.today)
quantity = models.PositiveSmallIntegerField(blank=True, default=1)
payment_made = models.IntegerField(default=0)
def __str__(self):
return self.invoiceOwner.name
Serializers file
class paymentInvoiceSerializer(serializers.ModelSerializer):
invoiceOwner = purchaserSerializer(many=False)
invoiceOwner = serializers.CharField(source='invoiceOwner.name')
class Meta:
model = paymentInvoice
fields = '__all__'
def create(self, validated_data):
purchaser_data = validated_data.pop("invoiceOwner")
purchaser, _ = Purchaser.objects.get_or_create(**purchaser_data).first()
validated_data.update({"invoiceOwner": purchaser})
return paymentInvoice.objects.create(**validated_data)
Views file
class PurchaserListCreateView(ListCreateAPIView):
serializer_class = purchaserSerializer
queryset = Purchaser.objects.all()
class paymentInvoiceListCreateView(ListCreateAPIView):
serializer_class = paymentInvoiceSerializer
queryset = paymentInvoice.objects.all().order_by('-date')
POST request in Postman for the model paymentInvoice
{
"invoiceOwner":"Becky",
"product": "Lix",
"quantity": 1,
"payment_made": 3000
}
My way is not working, i get the error from PostMan
MultipleObjectsReturned at /api/clients/invoice/
get() returned more than one Purchaser -- it returned 2!

Answer 1

You can first check for the record if not found then create one like below
def create(self, validated_data):
purchaser_data = validated_data.pop("invoiceOwner")
purchaser = Purchaser.objects.filter(**purchaser_data).first()
if purchaser is None:
purchaser = Purchaser.objects.create(**purchaser_data)
validated_data.update({"invoiceOwner": purchaser})
return paymentInvoice.objects.create(**validated_data)

Answer 2

Probably get_or_create should work also, just i think you should redifine get_or_create not create method .
def get_or_create(self):
purchaser_data = validated_data.pop("invoiceOwner")
purchase,created = Purchase.objects.get_or_create(**purchaser_data)
validated_data.update({"invoiceOwner": purchaser})
payment_invoice = paymentInvoice.objects.create(**validated_data)
return payment_invoice,created
in view :
model_serializer = paymentInvoiceSerializer(data=request.data)
if model_serializer.is_valid():
payment_invoice,_ = model_serializer.get_or_create()