django rest framework serializer add an auto generated field when getting data

Question

I want to automatically generate a field when data comes out of the database. Here is an example serializer I have:

class SaleSerializer(serializers.ModelSerializer):
    class Meta:
        model = Sale
        fields = '__all__'

I want this serializer to have two extra fields that dynamically generate one called username which has a string username of the user object and the other shop name that has the name of the shop object.

Here is the model of sale that the serializer uses:

class Sale(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    shop = models.ForeignKey(Shop, on_delete=models.DO_NOTHING)
    reciept = models.TextField()
    date_time = models.DateTimeField(auto_now_add=True)
    def __str__(self):
        return f"reciept number {self.pk}"

Answer 1

Try this (I have presumed that your Shop model has a field called name since you din't provide the class):

class SaleSerializer(serializers.ModelSerializer):
    username = serializers.SerializerMethodField()
    shop_name = serializers.SerializerMethodField()

    def get_username(self, obj):
       return obj.user.username
 
    def get_shop_name(self, obj):
       return obj.shop.name
 
    class Meta:
        model = Sale
        fields = ['user', 'shop', 'reciept', 'date_time', 'username', 'shop_name']

Answer 2

Assuming the name field stores the name of shop & user in their respective models. There is another you can achieve this using source argument in a serializer field apart from mentioned by RedWheelbarrow.

class SaleSerializer(serializers.ModelSerializer):
    username = serializers.Charfield(source='user.name')
    shopname = serializers.Charfield(source='shop.name')
    class Meta:
        model = Sale
        fields = '__all__'