在 Django ORM 的 Case-When 查询中，有没有办法使用任何用户定义的 Python 方法？

Question

I have defined a class along with its necessary methods, which is as following.

class Location(models.Model):
    latitude = models.DecimalField(max_digits=20, decimal_places=8, default=Decimal("0.00"), null=True)
    longitude = models.DecimalField(max_digits=20, decimal_places=8, default=Decimal("0.00"), null=True)

    @staticmethod
    def prepare_location_hotlink(latitude=None, longitude=None):
        returm mark_safe(s="<a class='inline-link' href='https://maps.google.com/maps?z=17&q={lat},{lng}' target='_blank'>{lat}, {lng}</a>".format(lat=latitude, lng=longitude))

    @classmethod
    def retrieve_location_data(cls):
        annotated_case_query = {
            "location": Case(
                When(Q(location__isnull=False, longitude__isnull=False),
                     then=cls.prepare_location_hotlink(latitude=F("latitude"), longitude=F("longitude"))
                ), default=Value("Not Available"), output_field=CharField()
            )
        }
        return [loc for loc in cls.objects.annotate(**annotated_case_query).values_list("location", flat=True)]

Here, in the retrieve_location_data method, I have tried to use a user-defined ( prepare_location_hotlink ) Python method in Case query to retrieve all of the location data as hotlinked. It seems not working in the above way. But I need to use the user-defined python method in the Case query anyway, as it retrieves and prepares data much faster. I already researched and read the Django documentation, but unfortunately, I could not find any solution for this case.

Is there any proper and accurate way on how to use any user-defined Python method in the Case query of Django ORM ?

Answer 1

Is there any proper and accurate way on how to use any user-defined Python method in the Case query of Django ORM?

No , since the database does not know anything about Django. It is just a program that inteprets queries to store/retrieve/aggregate data. But nevertheless, it is not a good idea to let the database do complex processing anyway. Databases are not designed for this.

You can simply perform the operations in the list comprehension:

from itertools import 

class Location(models.Model):
    latitude = models.DecimalField(
        max_digits=20,
        decimal_places=8,
        default=Decimal('0.00'),
        null=True
    )
    longitude = models.DecimalField(
        max_digits=20,
        decimal_places=8,
        default=Decimal('0.00'),
        null=True
    )

    @staticmethod
    def prepare_location_hotlink(long=None, lat=None):
        if :
            return 'Not Available'
        return mark_safe(f"<a class='inline-link' href='https://maps.google.com/maps?z=17&q={lat},{lng}' target='_blank'>{lat}, {lng}</a>")

    @classmethod
    def retrieve_location_data(cls):
        return list(starmap(
            
            cls.objects.values_list('longitude', 'latitude')
        )

Answer 2

You can use the Concat function to concatenate your string at database level, nevertheless, in this situation I would go for @Willem Van Onsem answer which is more readable and maintainable.

You can also wrap you queryset with list() to make it a list.

from django.db.models import Q, Case, When, Value, F
from django.db.models.functions import Concat

annotated_case_query = {
    "location": Case(
        When(Q(location__isnull=False, longitude__isnull=False),
            then=Concat(Value("<a class='inline-link' href='https://maps.google.com/maps?z=17&q="), F("latitude"), Value(","), F("longitude"), Value("' target='_blank'>"), F("latitude"), Value(", "), F("longitude"), Value('</a>'))
        ), default=Value("Not Available"), output_field=CharField()
    )
}
return list(cls.objects.annotate(**annotated_case_query).values_list("location", flat=True))