Where is it preferable to implement business logic in a Django application: in models, views, or other layers?

In Django, it is preferable to implement business logic in models and service layers rather than in views.

• Models: A good place for data-related logic — for example, methods for processing, validation, or computation at the object level.
• Service layer: For more complex business logic that does not directly relate to a single model, it is better to create separate classes or functions (e.g., in a services folder) to separate logic from models and views.
• Views: Should handle receiving requests, calling business logic, and returning responses, but not contain complex logic.

This approach improves code readability, testability, and maintainability.

Example of a method in a model:

class Order(models.Model):
    items = models.ManyToManyField(Item)
    status = models.CharField(max_length=20)

    def calculate_total(self):
        return sum(item.price for item in self.items.all())

For more complex operations, you can create a service:

# services/order_service.py

def process_order(order):
    if order.status != 'pending':
        raise ValueError('Order already processed')
    # business logic for processing the order
    order.status = 'processed'
    order.save()

The view calls the service:

from services.order_service import process_order

def order_view(request, order_id):
    order = Order.objects.get(id=order_id)
    process_order(order)
    return HttpResponse('Order processed')