[FIXED] django rest framework: How to validate a rule in serializers using a related field

Issue

I have these models:

product.py

class Product(models.Model):
    product_title = models.CharField(max_length=100)
    product_price = models.DecimalField(max_digits=12, decimal_places=2)
    product_multiple = models.PositiveIntegerField(blank=True, null=True)

order_item.py

class OrderItem(models.Model)
    order = models.ForeignKey(Order, on_delete=models.CASCADE, related_name='items', null=True, blank=True)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    price = models.FloatField(default=0)
    quantity = models.BigIntegerField(default=1)
    profitability = models.CharField(max_length=50, null=True, blank=True)

I want to validate in serializers.py if the order_item.quantity % product_multiple != 0

serializers.py

class OrderItemSerializer(serializers.ModelSerializer):

    class Meta:
        model = OrderItem
        fields = ('id', 'order', 'product', 'price',
                  'quantity', 'profitability')

    def validate_quantity(self, value):
        data = self.get_initial()
        quantity = data.get("quantity")

        # This is where the `product.product_multiple` value is needed
        if int(quantity) % product.product_multiple != 0:
            raise serializers.ValidationError("Quantity is not multiple")

        return value

How can I get the actual product_multiple inside the validate function?

Solution

As you are doing a validation that involves multiple fields, you should use the validate method as follows:

def validate(self, data):
    quantity = data.get('quantity')
    product = data.get('product')

    if int(quantity) % product.product_multiple != 0:
        raise serializers.ValidationError("Quantity is not multiple")
    return data

Answered By - Ozgur Akcali