[FIXED] How can I retrieve the value from a method in Django's model?

Issue

How to retrieve value from a method in Django's model I have two models Model1 and Model2, Model2 is related to Model1. The method is located in Model2. The method name is get_organization_name

class Model1(models.Model):
    code = models.CharField(max_length=5)
    random_id = models.IntegerField(primary_key=True)
    random_type = models.CharField(max_length=5)
    random_code = models.CharField(max_length=10)
    random_name = models.CharField(max_length=200)


class Model2(models.Model):
    id = models.AutoField(primary_key=True)
    claim_id = models.PositiveIntegerField()
    random = models.OneToOneField('Model1', on_delete=models.DO_NOTHING,
                         db_column='random_id', related_name='polis_journal')
    random_type = models.CharField(max_length=255)
    organization_id = models.PositiveIntegerField()

    def get_organization_name(self):
        return polis_organizations.get(self.organization_id, None)

I have tried following:

queryset = Model1.objects.all()
queryset = queryset.annotate(organization_name=F('polis_journal__get_organization_name')

Solution

You don't, or at least not without applying "tricks" to "translate" the method into a database expression.

In this specific case, we probably have a dictionary lookup, so we can do this with:

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

queryset = Model1.objects.annotate(
    organization_name=Case(
        *[
            When(polis_journal__organization_id=k, then=Value(v))
            for k, v in polis_organizations.items()
        ],
        default=Value(None)
    )
)

but not only is this ugly, and will this result in a large expression, it will perform dictionary lookups in linear time.

Usually one just stores the name of the organization in a model:

class Organization(models.Model):
    name = models.CharField(max_length=128, unique=True)


class Model2(models.Model):
    # …
    organization = models.ForeignKey(Organization, on_delete=models.PROTECT)

and then we can thus work with:

Model1.objects.annotate(organization_name=F('polis_journal__organization__name'))

Answered By - willeM_ Van Onsem