Say I have a model with a Things table and a table of relationships between the things called ThingRelations. It should not be possible to delete a Thing when there are ThingRelations that point to it. This is how I'm trying to implement that:

from django.db import models

class ThingRelation(models.Model):
    first_thing = models.ForeignKey('Thing', on_delete=models.PROTECT)
    second_thing = models.ForeignKey('Thing', on_delete=models.PROTECT)

class Thing(models.Model):
    name = CharField(max_length=260)

How do I automatically delete a Thing when there are no more ThingRelations pointing to it?

CodePudding user response：

You have such options:

• A routine. It can be made as a Command and something like crontab. Or it could be designed as a periodiq routine. This way you can repeatedly select all Thing models which have no relations with ThingRelation.
• signals.py action. This way when entry of ThingRelation is deleted, you should check both the first_thing and second_thing in order to know whether the have any more ThingRelation relations pointing to them.
• DB trigger (e.g. for PostgreSQL). The same idea as signal.py solution, but on DB level.

Which one should you choose? Depends on details of your exact objective. As for me, I use periodiq option on simple cases and DB trigger if I aim on high performance.