[FIXED] Django Filter by ForeignKey value

Issue

Suppose I have two models

class DocumentType(BaseModel):
    name = models.CharField(max_length=128)
    code = models.CharField(max_length=128)
    exp_days = models.PositiveIntegerField("Remind me before (Days)", blank=True, null=True)

    def __str__(self):
        return str(self.name)


class Document(BaseModel):
    type = models.ForeignKey("masters.DocumentType", on_delete=models.PROTECT)
    date_of_expiry = models.DateField(blank=True, null=True)
    attachment = models.FileField(blank=True, null=True)
    
    def __str__(self):
        return str(self.employee)

I need to get the documents that will expire in the next {exp_days} mentioned in the type.

documents = Document.objects.filter(date_of_expiry__lte=today+timedelta(days=30))

The above filter will return the documents expiring in the next 30 days. But how to apply the exact same condition mentioned above

Solution

You can use F() object that represents the value of a model field. so to reference exp_days from related model you can use F('type__exp_days')

An answer from Lutz Prechelt shows how to move the F() expression outside the timedelta.

Here is the modified code

from django.db.models import F

documents = Document.objects.filter(date_of_expiry__lte=today+timedelta(days=1)*F('type__exp_days'))

As mentioned here, this will work with PostgreSQL but, might not with other DBs (Not confirmed).

Answered By - Asad Mahmood