Where can the business logic of the application be…

Business logic can be located in the following places:

In data models: Most often, when the logic is closely related to data and its state (for example, field validation, calculation of derived values).

# Example of storing business logic in a model
class Order:
    def __init__(self, items):
        self.items = items

    def calculate_total(self):
        # Business logic for calculating the total order cost
        total = sum(item.price for item in self.items)
        return total

In service layers: Separate modules or classes containing operations that combine multiple models or perform more complex business processes. This is the preferred approach for complex applications, following clean architecture principles.

# Example of a service layer
class OrderService:
    def create_order(self, user_id, items_data):
        # Get user data, create Item objects, create Order
        # There can be a lot of business logic here: checking product availability,
        # calculating discounts, notifications, etc.
        pass

    def process_payment(self, order_id, payment_details):
        # Logic for processing payment for an order
        pass

In controllers or views: Highly discouraged, only for very simple logic directly related to request processing and response formation. Violates the separation of concerns principle.

# Anti-pattern: business logic in controller (to be avoided)
def process_order_request(request):
    user_id = request.user.id
    items = request.GET.getlist('items')
    # Complex order creation logic should not be implemented here,
    # better to move it to a service or model.
    # order = create_order_logic(user_id, items)
    pass

The choice of storage location depends on the complexity of the logic, the size of the application, and the architecture used. It is recommended to aim for separating business logic into distinct layers (services, domain objects) to improve testability, maintainability, and modularity of the code.

Where can the business logic of the application be stored?