Separation of business logic and data access in django Ask Question

It seems like you are asking about the difference between the data model and the domain model – the latter is where you can find the business logic and entities as perceived by your end user, the former is where you actually store your data.

Furthermore, I've interpreted the 3rd part of your question as: how to notice failure to keep these models separate.

These are two very different concepts and it's always hard to keep them separate. However, there are some common patterns and tools that can be used for this purpose.

About the Domain Model

The first thing you need to recognize is that your domain model is not really about data; it is about actions and questions such as "activate this user", "deactivate this user", "which users are currently activated?", and "what is this user's name?". In classical terms: it's about queries and commands.

Thinking in Commands

Let's start by looking at the commands in your example: "activate this user" and "deactivate this user". The nice thing about commands is that they can easily be expressed by small given-when-then scenario's:

given an inactive user
when the admin activates this user
then the user becomes active
and a confirmation e-mail is sent to the user
and an entry is added to the system log
(etc. etc.)

このようなシナリオは、単一のコマンドによってインフラストラクチャのさまざまな部分（この場合は、データベース（何らかの「アクティブ」フラグ）、メール サーバー、システム ログなど）がどのように影響を受けるかを確認するのに役立ちます。

このようなシナリオは、テスト駆動開発環境の設定にも非常に役立ちます。

そして最後に、コマンドで考えることは、タスク指向のアプリケーションを作成するのに非常に役立ちます。ユーザーはこれを高く評価するでしょう :-)

コマンドの表現

Django では、コマンドを表現する簡単な方法が 2 つ用意されています。どちらも有効なオプションであり、2 つのアプローチを混在させることは珍しくありません。

サービス層

サービスモジュールはすでに@Hedde による説明ここでは個別のモジュールを定義し、各コマンドは関数として表されます。

サービス.py

def activate_user(user_id):
    user = User.objects.get(pk=user_id)

    # set active flag
    user.active = True
    user.save()

    # mail user
    send_mail(...)

    # etc etc

class ActivateUserForm(forms.Form):

    user_id = IntegerField(widget = UsernameSelectWidget, verbose_name="Select a user to activate")
    # the username select widget is not a standard Django widget, I just made it up

    def clean_user_id(self):
        user_id = self.cleaned_data['user_id']
        if User.objects.get(pk=user_id).active:
            raise ValidationError("This user cannot be activated")
        # you can also check authorizations etc. 
        return user_id

    def execute(self):
        """
        This is not a standard method in the forms API; it is intended to replace the 
        'extract-data-from-form-in-view-and-do-stuff' pattern by a more testable pattern. 
        """
        user_id = self.cleaned_data['user_id']

        user = User.objects.get(pk=user_id)

        # set active flag
        user.active = True
        user.save()

        # mail user
        send_mail(...)

        # etc etc

User.objects.filter(active=True)

<h1>Welcome, {{ user|friendly_name }}</h1>

@register.filter
def friendly_name(user):
    return remote_api.get_cached_name(user.id)

def inactive_users():
    return User.objects.filter(active=False)


def users_called_publysher():
    for user in User.objects.all():
        if remote_api.get_cached_name(user.id) == "publysher":
            yield user 

class InactiveUserManager(models.Manager):
    def get_queryset(self):
        query_set = super(InactiveUserManager, self).get_queryset()
        return query_set.filter(active=False)

class InactiveUser(User):
    """
    >>> for user in InactiveUser.objects.all():
    …        assert user.active is False 
    """

    objects = InactiveUserManager()
    class Meta:
        proxy = True

class InactiveUserDistribution(models.Model):
    country = CharField(max_length=200)
    inactive_user_count = IntegerField(default=0)

class ActivateUserForm(forms.Form):
    # see above
   
    def execute(self):
        # see above
        query_model = InactiveUserDistribution.objects.get_or_create(country=user.country)
        query_model.inactive_user_count -= 1
        query_model.save()

user_activated = Signal(providing_args = ['user'])
user_deactivated = Signal(providing_args = ['user'])

class ActivateUserForm(forms.Form):
    # see above
   
    def execute(self):
        # see above
        user_activated.send_robust(sender=self, user=user)

class InactiveUserDistribution(models.Model):
    # see above

@receiver(user_activated)
def on_user_activated(sender, **kwargs):
        user = kwargs['user']
        query_model = InactiveUserDistribution.objects.get_or_create(country=user.country)
        query_model.inactive_user_count -= 1
        query_model.save()