[Answer]-How do I restrict a Django model to just one of several possible relationships?

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You could have the relationship on the FinancialTransaction using a Generic foreign key.

https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic

class FinatialTransation(models.Model):
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_type', 'object_id')

Then the relationship exists in one place and there can only be 1.

Then from the FinancialTransaction you check the object ID and the objects ContentType and look it up accordingly.

ft = FinancialTransaction.objects.get(...)
thing = ft.content_type.get_object_for_this_type(id=ft.object_id)

Additionally you can then limit the GenericForeignKey to certain content types with:

class FinatialTransation(models.Model):
    limit = models.Q(
        models.Q(app_label='yourappsname', model='ThingOne') | models.Q(app_label='yourappsname', model='ThingTwo') | models.Q(app_label='yourappsname', model='ThingThree')
    )
    content_type = models.ForeignKey(ContentType, limit_choices_to=limit)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_type', 'object_id')
👤rockingskier

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