Organize Logic

Understanding Services, Querysets, and Managers in Django, Use Cases and Best Practices

In Django, there are several tools and patterns available to help you organize your application's logic and interact with the database. Three important concepts are services, querysets, and managers. Each of these has its own purpose and best practices for usage. In this article, we will explore where to use services, querysets, and managers in Django and provide guidance on their appropriate use cases.

Services

Services in Django encapsulate business logic and handle complex operations that involve multiple models or require additional processing beyond basic database operations. They provide a way to separate the application's business logic from the views and models, promoting a more modular and maintainable codebase. Here are some common scenarios where services are useful:

• Complex data processing
• Integration with external systems
• Orchestrating model interactions
• Enforcing business rules
• Reusability and modularity

Querysets

Querysets in Django provide a powerful API for querying the database and retrieving objects. They are used to construct database queries and represent a collection of model instances that match certain criteria. Querysets are commonly used in the following situations:

• Retrieving specific sets of objects from the database
• Filtering and ordering objects
• Performing aggregations and annotations
• Chaining multiple query operations together
• Lazy evaluation and optimization of queries

Managers

Managers in Django are responsible for retrieving objects from the database and encapsulating common query operations. They provide a convenient interface for interacting with the underlying queryset. Managers are typically defined on the model level and allow you to create custom querysets and define model-specific operations. Use managers in the following scenarios:

• Defining custom query methods specific to a model
• Encapsulating common query patterns
• Implementing model-specific logic
• Modifying the default behavior of object retrieval

Where to use QuerySets and Managers?

Both QuerySets and managers are used in Django to perform database queries, but they serve different purposes.

A QuerySet is a collection of database objects (model instances) that can be queried and filtered. It represents the results of a database query and provides a chainable API for working with the data. You typically use QuerySets in your views, templates, and other parts of your code to retrieve, filter, and manipulate data from the database.

A Manager, on the other hand, is the interface through which you interact with the database at the model level. It provides methods for creating, retrieving, updating, and deleting model instances. Managers are defined in the model class and allow you to customize the way database queries are executed and results are returned. You can add custom methods to managers to encapsulate common query logic or create custom QuerySets.

In summary, you use QuerySets when you want to perform database queries and retrieve data, while managers provide an interface for working with model-level operations and customizing query behavior. Managers can have methods that return QuerySets to provide a higher level of abstraction and encapsulate common query patterns.

In most cases, you will use QuerySets more frequently in your views and templates to retrieve and manipulate data, while managers are used to customize the behavior of model-level operations and provide reusable query logic.

Where to use service?

Services in Django are typically used to encapsulate business logic and handle complex operations that involve multiple models or require additional processing beyond basic database operations. They provide a way to separate the application's business logic from the views and models, promoting a more modular and maintainable codebase.

Here are some scenarios where you might consider using a service:

1. Complex Data Processing: If you need to perform complex calculations, data transformations, or other operations that go beyond simple database queries or model manipulations, a service can encapsulate this logic.
2. Integration with External Systems: If your application needs to interact with external APIs, services can handle the communication, data transformation, and error handling related to those integrations.
3. Orchestrating Model Interactions: When you have multiple models that need to be created, updated, or deleted in a specific sequence or with certain validations, a service can coordinate these interactions to ensure consistency and integrity.
4. Business Rule Enforcement: If your application has specific business rules or policies that need to be enforced during data operations, services can encapsulate and enforce those rules.
5. Reusability and Modularity: Services can be designed to be reusable across different parts of your application, promoting modularity and reducing code duplication.

When deciding whether to use a service, consider the complexity and scope of the operation, whether it involves multiple models or external systems, and whether it aligns with the responsibilities of your views and models. If the logic goes beyond the basic responsibilities of views and models and requires additional processing or coordination, a service can be a good fit.

Best Practices

• Services, querysets, and managers serve different purposes, so it's important to use them appropriately based on the specific requirements of your application.
• Services should be used to encapsulate complex business logic and orchestrate interactions between models or external systems.
• Querysets are ideal for constructing database queries and performing filtering, ordering, and aggregations.
• Managers provide a convenient interface for model-specific query operations and customization.
• Avoid mixing business logic directly in models or views. Instead, delegate complex operations to services and leverage querysets and managers for efficient data retrieval and manipulation.

---

Understanding when and where to use services, querysets, and managers in Django is crucial for building maintainable and scalable applications. By leveraging these tools effectively, you can achieve a separation of concerns, improve code organization, and enhance the overall flexibility and maintainability of your Django projects.

Remember to carefully evaluate your application's requirements and choose the appropriate tool for each specific scenario. Following best practices will help you create clean and modular code that is easier to understand, test, and maintain.

---

Example of Saleor way

In Saleor, the QuerySet class is extended to provide custom query behavior similar to managers in Django. Saleor uses a pattern called "custom query sets" where custom methods are added to the QuerySet class to encapsulate specific query logic. This approach allows for a more expressive and reusable query API.

Here's an example code snippet from Saleor to demonstrate the usage of custom query sets:

from django.db import models


class ProductQuerySet(models.QuerySet):
    def available(self):
        return self.filter(is_available=True)

    def priced_under(self, price):
        return self.filter(price__lt=price)


class Product(models.Model):
    name = models.CharField(max_length=255)
    price = models.DecimalField(max_digits=10, decimal_places=2)
    is_available = models.BooleanField(default=True)

    objects = ProductQuerySet.as_manager()

    def __str__(self):
        return self.name

In the above example:

• The ProductQuerySet class extends the QuerySet class and defines custom query methods available and priced_under.
• The Product model includes the objects attribute, which is an instance of the ProductQuerySet class created with as_manager() method. This makes the custom query methods accessible directly from the model manager.

Using custom query sets in Saleor allows for more concise and expressive queries, making it easier to work with complex filtering and retrieval requirements specific to the application. It provides a convenient way to encapsulate query logic and reuse it across different parts of the codebase.

In Saleor, the concept of services is used to encapsulate business logic and handle complex operations that go beyond simple database queries. Services are typically used to orchestrate multiple actions, handle transactions, perform validations, and interact with external systems or APIs.

The services in Saleor are usually implemented as separate modules or packages within the application. They are organized based on the domain or functionality they serve. Here's an example of a service in Saleor:

from saleor.order.models import Order
from saleor.payment.models import Payment

class OrderService:
    @classmethod
    def create_order(cls, user, cart, billing_address, shipping_address):
        order = Order.objects.create(
            user=user,
            billing_address=billing_address,
            shipping_address=shipping_address,
            status=OrderStatus.NEW,
        )

        # Add products from the cart to the order
        for item in cart.items:
            product = item.product
            order.lines.create(product=product, quantity=item.quantity, price=item.price)

        # Calculate the total amount
        order.total = order.get_total()

        # Perform payment
        payment = Payment.objects.create(order=order, amount=order.total)
        payment.process()

        return order

In the above example, the OrderService class provides a create_order method that handles the creation of a new order. It takes various parameters such as the user, cart, billing address, and shipping address. Within the method, it performs multiple actions such as creating the order, adding products from the cart, calculating the total amount, and processing the payment.

By using services, Saleor keeps the business logic separate from the models and views, promoting a more modular and maintainable code structure. Services help in encapsulating complex operations, improving code organization, reusability, and testability. They also allow for easier integration with external systems and APIs by providing a clear interface for interacting with them.

Fat Model and Thin View

Saleor follows the principle of having a "fat model" and "thin view" architecture. This means that the majority of business logic and data manipulation is handled within the model layer, while the views remain lightweight and focused on handling HTTP requests and responses.

In Saleor, the models contain the core business logic and handle operations such as creating orders, processing payments, managing inventory, and applying discounts. The models encapsulate the logic and provide methods for performing these operations, making them more self-contained and reusable.

On the other hand, the views in Saleor are responsible for handling HTTP requests, validating input data, and returning appropriate responses. They act as an interface between the client and the models, delegating the actual business logic to the model methods. Views in Saleor are often implemented as Django's class-based views or Django REST Framework's viewsets.

By adhering to the "fat model" and "thin view" principle, Saleor achieves a separation of concerns, improves code maintainability, and allows for easier testing and reusability of the model logic. It promotes a structured and organized codebase that is focused on the core domain logic of the application.