django model method to return queryset with annotate

Question

i am trying to define a field which being calculated based on user_type, I did the following method but its not working, if anyone can advise me to the correct approach.

so what i am trying to achieve here is to construct a method such as <user>.book_limit to return the maximum number of allowed books based on user_type

#models.py 

class UserProfile(models.Model):
    class UserType(models.TextChoices):
        FREE = 'FR', 'FREE'
        BASIC = 'BS', 'BASIC'
        PREMIUM = 'PR', 'PREMIUM'

    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name='profile')
    bio = models.CharField(max_length=255)
    created_on = models.DateTimeField(auto_now_add=True)
    last_updated = models.DateTimeField(auto_now=True)
    user_type = models.CharField(max_length=2, choices=UserType.choices, default=UserType.FREE)

    def __str__(self):
        return self.user.username

    @property
    def borrow_limit(self):
        return UserProfile.objects.annotate(book_limit=Case(
            When(user_type=UserProfile.UserType.FREE, then=Value('1')),
            When(user_type=UserProfile.UserType.BASIC, then=Value('2')),
            When(user_type=UserProfile.UserType.PREMIUM, then=Value('5')),
            default=Value('1'), output_field=IntegerField))

Take a look at Django managers: https://docs.djangoproject.com/en/3.0/topics/db/managers/ — Willem Van Onsem, Jun 19 '20 at 08:07

score 2 · Accepted Answer · answered Jun 19 '20 at 10:17

You can use a manager to each time annotate the value:

from django.db.models import Case, IntegerField, Value, When

class UserProfileManager(models.Manager):

    def get_queryset(self, *args, **kwargs):
        return super().get_queryset(*args, **kwargs).annotate(
            book_limit=Case(
                When(user_type=UserProfile.UserType.FREE, then=Value(1)),
                When(user_type=UserProfile.UserType.BASIC, then=Value(2)),
                When(user_type=UserProfile.UserType.PREMIUM, then=Value(5)),
                default=Value(1),
                output_field=IntegerField()
            )
        )

class UserProfile(models.Model):
    class UserType(models.TextChoices):
        FREE = 'FR', 'FREE'
        BASIC = 'BS', 'BASIC'
        PREMIUM = 'PR', 'PREMIUM'

    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name='profile')
    bio = models.CharField(max_length=255)
    created_on = models.DateTimeField(auto_now_add=True)
    last_updated = models.DateTimeField(auto_now=True)
    user_type = models.CharField(max_length=2, choices=UserType.choices, default=UserType.FREE)
    objects = UserProfileManager()

but can you please explain how this works, why we add ars and kwargs for the `get_queryset` — Ahmed Al-Haffar, Jun 19 '20 at 16:05
@AhmedAl-Haffar: it means we pass all *positional* and *named* parameters to the super call. If you later for example add a mixin, then it can introuduce/consume parameters. See for example: https://stackoverflow.com/a/8973101/67579 — Willem Van Onsem, Jun 19 '20 at 17:22

score 1 · Answer 2 · answered Jun 19 '20 at 08:17

1

You can customize your initial queryset like described here.

answered Jun 19 '20 at 08:17

Ali Asgari

801
7
18

django model method to return queryset with annotate

2 Answers2

Linked