django-db-cascade-2

Installation for Django 3

pip install django-db-cascade-2

Installation for Django 2

pip install django-db-cascade-2==0.2.3

settings.py:

DATABASES = {
    'default': {
        'ENGINE': 'django_db_cascade.backends.postgresql_psycopg2',
        # ... etc ...
    }
}

Usage

from django.db import models
from django_db_cascade.fields import ForeignKey, OneToOneField
from django_db_cascade.deletions import DB_CASCADE

class Thing(Common):
    account = ForeignKey('self', DB_CASCADE)

Caveats

• DB_CASCADE only supports Postgres
• DB_CASCADE does not support django on_delete signals
• DB_CASCADE will not cascade delete multiple inherited tables as expected
• DB_CASCADE will not trigger CASCADE on another model. E.g. Model A points to model B, via DB_CASCADE. Model B points to model C, via CASCADE. A will cascade delete B, B will django delete C, but deleting A will not delete C!
• DB_CASCADE on a ManyToMany of A <---> B, only A_B set records will be cascade deleted (deleting A will not delete B)

How it works

1. Minimal subclassing of the django postgresql backend and the django ForeignKey field
2. Added a new possible value for ForeignKey's on_delete kwarg, called DB_CASCADE
3. When you use DB_CASCADE, the migration framework will recognize a change, and write new sql
4. example SQL generated:

   ALTER TABLE mytable ADD CONSTRAINT myconstraint FOREIGN KEY (mycolumn)
   REFERENCES myothertable myothercolumn ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED
   

Future proof

If, and when, DB_CASCADE ever gets into django, editing these generated migrations should be very easy.

Generated migrations:

migrations.AlterField(
    model_name='modelname',
    name='fieldname',
    field=django_db_cascade.fields.ForeignKey(on_delete=django_db_cascade.deletions.DB_CASCADE)
)

Changing them over, if django ever handles DB_CASCADE natively, might look like:

migrations.AlterField(
    model_name='modelname',
    name='fieldname',
    field=django.db.models.ForeignKey(on_delete=models.DB_CASCADE)
)

Ticket

The ticker where django has discussed bringing DB_CASCADE to django: https://code.djangoproject.com/ticket/21961