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
29358	Add a system check to prohibit models with more than one primary_key field	zt_initech	nobody	"If a Django Model has two Fields with `primary_key=True`, Django will carry on without complaining at all, but silently corrupt data because the WHERE clause of UPDATE statement generated for a single model instance's `save()` will only use the first field that has `primary_key=True`. Same for `refresh_from_db()` and other things.

I know that the documentation at https://docs.djangoproject.com/en/dev/ref/models/fields/#primary-key says ""Only one primary key is allowed on an object."".

I know about https://code.djangoproject.com/ticket/373 and this is not a duplicate of it.

I know that migrate will fail with `django.db.utils.OperationalError: table ""%s"" has more than one primary key`, but people use Django with pre-existing database tables.

This ticket is only about making Django fail when trying to load a Model with more than one primary key field, so that people won't try to use such a database table with Django.

If there is some need to let Django access such tables, at least add a warning.

Steps to reproduce:

{{{#!bash
mkdir compositepk
cd compositepk
rm -rf venv compositepk
virtualenv -p `which python3.6` venv
source venv/bin/activate
pip install Django
django-admin startproject compositepk
cd compositepk/
./manage.py startapp app
echo ""INSTALLED_APPS = INSTALLED_APPS + ['app']"" >> compositepk/settings.py
VAR1=$(cat <<EOF
from django.db import models

class M(models.Model):
    f1 = models.IntegerField(primary_key=True)
    f2 = models.IntegerField(primary_key=True)
    f3 = models.IntegerField()
EOF
)
echo ""${VAR1}"" > app/models.py
unset VAR1
./manage.py migrate contenttypes
./manage.py migrate auth
./manage.py migrate admin
./manage.py migrate sessions
echo ""create table app_m (f1 integer not null, f2 integer not null, f3 integer, primary key (f1, f2));"" | sqlite3 db.sqlite3
echo ""insert into django_migrations (app, name, applied) values ('app', '0001_initial', datetime());"" | sqlite3 db.sqlite3
./manage.py makemigrations
./manage.py migrate
VAR1=$(cat <<EOF
from app.models import M
m = M.objects.create(f1=1, f2=1)
m.refresh_from_db()
m.f3 = 42
m.save()
from django.db import connection
print(connection.queries[-3]['sql'])
print(connection.queries[-1]['sql'])
EOF
)
echo ""${VAR1}"" | ./manage.py shell
unset VAR1
deactivate
cd ..
}}}

You see it prints:

{{{
SELECT ""app_m"".""f1"", ""app_m"".""f2"", ""app_m"".""f3"" FROM ""app_m"" WHERE ""app_m"".""f1"" = 1
UPDATE ""app_m"" SET ""f3"" = 42 WHERE ""app_m"".""f1"" = 1
}}}"	Cleanup/optimization	closed	Core (System checks)	2.0	Normal	fixed			Accepted	1	0	0	0	0	0