Django ORM specifc query

Question

I have 2 django models:

class TelegramUser(models.Model):
    telegram_id = models.IntegerField()
    telegram_username = models.TextField(max_length=255, default=None, blank=True, null=True)
    first_name = models.TextField(max_length=255, default=None, blank=True, null=True)
    last_name = models.TextField(max_length=255, default=None, blank=True, null=True)
    has_blocked = models.BooleanField(default=False)

class DutyDate(models.Model):
    date = models.DateField(default=None, blank=True, null=True)
    telegram_user = models.ForeignKey(TelegramUser, on_delete=models.SET_NULL, null=True)
    group = models.ForeignKey(UserGroup, on_delete=models.CASCADE, null=True)
    mark = models.FloatField(max_length=255, default=0)
    is_open = models.BooleanField(default=True)

I need to get all users, who has less than 3 assigned DutyDates. Can I do it with one query and how?

willeM_ Van Onsem · Accepted Answer · 2019-12-25 09:30:52Z

1

Yes. We first count the number of related DutyDates, and then we filter on that count:

from django.db.models import Count

TelegramUser.objects.annotate(
    nduty=Count('dutydate')
).filter(
    nduty__lt=3
)

Here the __lt lookup [Django-doc] filters such that nduty is (strictly) less than 3. If you want to retain TelegramUsers that have three related DutyDates as well, you can use the __lte lookup [Django-doc].

answered Dec 25, 2019 at 9:30

willeM_ Van Onsem

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