Dry ORM

Code

Result

no cache

from django.db import models
from django.db.models import Q, CheckConstraint

class Place(models.Model):
    name = models.CharField(max_length=50)

class News(models.Model):
    title = models.CharField(max_length=50)

class File(models.Model):
    file = models.CharField(max_length=200)
    subject = models.CharField(max_length=50)
    file_type = models.CharField(max_length=50)

place = models.ForeignKey(
        Place,
        on_delete=models.CASCADE,
        related_name="files",
        null=True,
        blank=True,
    )

news = models.ForeignKey(
        News,
        on_delete=models.CASCADE,
        related_name="files",
        null=True,
        blank=True,
    )

class Meta:
        constraints = [
            CheckConstraint(
                check=(
                    # Either place is set and news is null
                    (Q(place__isnull=False) & Q(news__isnull=True)) |
                    # Or news is set and place is null
                    (Q(place__isnull=True) & Q(news__isnull=False))
                ),
                name="only_one_fk_filled"
            )
        ]

def run():
    place = Place.objects.create(name="Park")
    news = News.objects.create(title="Breaking News")

valid1 = File.objects.create(
        file="a.txt",
        subject="example",
        file_type="txt",
        place=place
    )
    print(f'Created with place: {valid1.id}')

valid2 = File.objects.create(
        file="b.txt",
        subject="example",
        file_type="txt",
        news=news
    )
    print(f'Created with news: {valid2.id}')

try:
        File.objects.create(
            file="c.txt",
            subject="example",
            file_type="txt",
            place=place,
            news=news
        )
    except Exception as e:
        print(f'Invalid (both FKs set): {e}')

Output

No output.

Queries

Queries Q (0)

No queries.