Django Rest Framework How to get instance foreign key?

Question

Django

I will explain the essence, since the vocabulary is not rich in terms.
When creating a new App_form, a ChosenProduct
is added that contains a product from Warehouse and the amount of product added to ChosenProduct is reduced. Now, when App_form is changed, that is, the value of terminated is changed to true by the patch method , this App_form gets, say, into the archive, and at the same time, the products from chosen_products should return to the Warehouse as a quantity , and after chosen_products is cleared.

class WarehouseSerializer(serializers.ModelSerializer):
    category_name = serializers.ReadOnlyField(
        source='category_product.category_name')
    posted_user = serializers.ReadOnlyField(
        source='posted_user.username')
    class Meta:
        model = Warehouse
        fields = ['id', 'category_product', 'category_name', 'condition',
                  'product_name', 'amount', 'barcode', 'f_price', 'created_at', 'updated_at', 'posted_user']

class ChosenProductSerializer(serializers.ModelSerializer):
    product_info = WarehouseSerializer(source='product', read_only=True)
    period_info = Product_periodSerializer(source='period', read_only=True)
    class Meta:
        model = ChosenProduct
        exclude = ('app_form',)

class App_formSerializer(serializers.ModelSerializer):
    chosen_products = ChosenProductSerializer(many=True)

  @transaction.atomic
    def create(self, validated_data):
        chosen_products_data = validated_data.pop('chosen_products')
        app_form = App_form.objects.create(**validated_data)
        for chosen_product_data in chosen_products_data:
            chosen_product = ChosenProduct.objects.create(app_form=app_form, **chosen_product_data)
            product = chosen_product.product
            product.amount = product.amount - chosen_product.quantity
            product.save()
        return app_form

    def update(self, instance, validated_data):
        instance.terminated = validated_data.get('terminated', instance.terminated)
        
        if instance.terminated == True :
              
            print('-----------TRUE--------------------')
            print(instance.chosen_products)
            print('-----------PRINT--------------------')

        instance.save()
        return instance

    class Meta:
        model = App_form
        fields = '__all__'

OUT:

-----------TRUE--------------------
creditapi.ChosenProduct.None
-----------PRINT--------------------

Answer 1

Implement a model method that does what you want and just call it in update.