id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
31343	Foreign key constraint to MTI child model is lost if the child model's parent was changed.	Antonis Christofides	nobody	"= Steps to reproduce

1. In a project using PostgreSQL, create an app `places` with the following models:
{{{
from django.db import models


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


class Restaurant(Place):
    serves_hot_dogs = models.BooleanField(default=False)


class MenuItem(models.Model):
    restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
}}}

2. Run `./manage.py makemigrations`. It will create `0001_initial.py`.

3. Change the models to this:
{{{
from django.db import models


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


class CommercialPlace(Place):
    business_name = models.CharField(max_length=50)


class Restaurant(CommercialPlace):
    serves_hot_dogs = models.BooleanField(default=False)


class MenuItem(models.Model):
    restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
}}}

The difference is that `Restaurant` now inherits the new `CommercialPlace`, whereas before it was directly inheriting `Place`. The `MenuItem` model is untouched.

4. Execute `/.manage.py makemigrations`.

At this stage it will not know what to do (""You are trying to add a non-nullable field 'commercialplace_ptr' to restaurant without a default""). Select option 1 (provide a one-off default) and enter 0 as the default value.

(This doesn't make sense, but it doesn't matter. What should happen here is for `commercialplace_ptr` to get the value `place_ptr` previously had, but Django can't figure this out on its own, so the developer will need to write it in the migration. This has nothing to do with the bug, so in this example we can ignore this problem.)

Migration 0002_auto_something.py will be created as a result of this step.

5. Execute `./manage.py sqlmigrate places 0002`.

= Result

The sqlmigrate output contains, among other things, this:

{{{
ALTER TABLE ""places_restaurant"" DROP COLUMN ""place_ptr_id"" CASCADE;
}}}

As a result of the CASCADE, the foreign key constraint from `MenuItem.restaurant` will be dropped. **No constraint is created in its place.**"	Bug	closed	Migrations	dev	Normal	duplicate			Unreviewed	0	0	0	0	0	0