Django Styleguide

Django styleguide used in HackSoft projects.

Expect often updates as we discuss & decide upon different things.

Overview

In Django, business logic should live in:

• Model properties (with some exceptions).
• Model clean method for additional validations (with some exceptions).
• Services - functions, that take care of code written to the database.
• Selectors - functions, that take care of code taken from the database.

In Django, business logic should not live in:

• APIs and Views.
• Serializers and Forms.
• Form tags.
• Model save method.

Model properties vs selectors:

• If the model property spans multiple relations, it should better be a selector.
• If a model property, added to some list API, will cause N + 1 problem that cannot be easily solved with select_related, it should better be a selector.

Services

A service is a simple function that:

• Lives in your_app/services.py module
• Takes keyword-only arguments
• Is type-annotated (even if you are not using mypy at the moment)
• Works mostly with models & other services and selectors
• Does business logic - from simple model creation to complex cross-cutting concerns, to calling external services & tasks.

An example service that creates an user:

def create_user(
    *,
    email: str,
    name: str
) -> User:
    user = User(email=email)
    user.full_clean()
    user.save()

    create_profile(user=user, name=name)
    send_confirmation_email(user=user)

    return user

As you can see, this service calls 2 other services - create_profile and send_confirmation_email

Selectors

A selector is a simple function that:

• Lives in your_app/selectors.py module
• Takes keyword-only arguments
• Is type-annotated (even if you are not using mypy at the moment)
• Works mostly with models & other services and selectors
• Does business logic around fetching data from your database

An example selector that list users from the database:

def get_users(*, fetched_by: User) -> Iterable[User]:
    user_ids = get_visible_users_for(user=fetched_by)

    query = Q(id__in=user_ids)

    return User.objects.filter(query)

As you can see, get_visible_users_for is another selector.

APIs & Serializers

When using services & selectors, all of your APIs should look simple & the same.

General rules for an API is:

• Do 1 API per operation. For CRUD on a model, this means 4 APIs.
• Use the most simple APIView or GenericAPIView
• Use services / selectors & don't do business logic in your API.
• Use serializers for fetching objects from params - passed either via GET or POST
• Serializer should be nested in the API and be named either InputSerializer or OutputSerializer
  • OutputSerializer can subclass ModelSerializer, if needed.
  • InputSerializer should always be a plain Serializer
  • Reuse serializers as little as possible
  • If you need a nested serializer, use the inline_serializer util

An example list API

class CourseListApi(SomeAuthenticationMixin, APIView):
    class OutputSerializer(serializers.ModelSerializer):
        class Meta:
            model = Course
            fields = ('id', 'name', 'start_date', 'end_date')

    def get(self, request):
        courses = get_courses()

        data = self.OutputSerializer(courses, many=True)

        return Response(data)

An example detail API

class CourseDetailApi(SomeAuthenticationMixin, APIView):
    class OutputSerializer(serializers.ModelSerializer):
        class Meta:
            model = Course
            fields = ('id', 'name', 'start_date', 'end_date')

    def get(self, request, course_id):
        course = get_course(id=course_id)

        data = self.OutputSerializer(course)

        return Response(data)

An example create API

class CourseCreateApi(SomeAuthenticationMixin, APIView):
    class InputSerializer(serializers.Serializer):
        name = serializers.CharField()
        start_date = serializers.DateField()
        end_date = serializers.DateField()

    def post(self, request):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        create_course(**serializer.validated_data)

        return Response(status=status.HTTP_201_CREATED)

An example update API

class CourseUpdateApi(SomeAuthenticationMixin, APIView):
    class InputSerializer(serializers.Serializer):
        name = serializers.CharField(required=False)
        start_date = serializers.DateField(required=False)
        end_date = serializers.DateField(required=False)

    def post(self, request, course_id):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        update_course(course_id=course_id, **serializer.validated_data)

        return Response(status=status.HTTP_200_OK)

Nested serializers

In case you need to use a nested serializer, you can do the following thing:

class Serializer(serializers.Serializer):
    weeks = inline_serializer(many=True, fields={
        'id': serializers.IntegerField(),
        'number': serializers.IntegerField(),
    })

The implementation of inline_serializer can be found in utils.py in this repo.

Inspiration

The way we do Django is inspired by the following things:

• The general idea for separation of concerns
• Boundaries by Gary Bernhardt
• Rails service objects