hpw to create related model in django rest framework logic

1class OrderSerializer(serializers.ModelSerializer):
2    class Meta:
3        model = Order
4        fields = (
5            'id',
6            'total',
7            'discount',
8        )
9
10    def calculate_discount(self, whatever_params):
11        # calculate discount if you need... and return it
12
13    def calculate_tax(self, whatever_params):
14        # calculate tax amount if you need...and return it
15
16    def calculate_grand_total(self, whatever_params):
17        # calculate grand total if you need and return it
18
19    def create(self, validated_data):
20        # You can make an order by applying 
21        # some logic in the calculation method.
22        # Maybe by adding a bit of the context 
23        # you sent as parameters from view.