This project is a test to show issues during python manage.py migrate when changing the field type for a field with a constraint.

Related tickets: 28305, 30152.

It seems 28305 does not fix this specific problem since the same error message can still occur. 30152 seems to fix the related_name='+' issue and we then end up with an exception:

django.db.utils.IntegrityError: (1215, 'Cannot add foreign key constraint')

Why does this happen ? The two tables have incompatible types for the same field. MySQL (tested on 5.7) needs to have to same type for both fields to be able to add a constraint.

How to test the bug

The bug is demonstrated by running the test suite. This will attempt to create a test db by running the migrations and should fail in git branches cannot-add-foreign-key-constraint and cannot-change-column-used-in-a-foreign-key-constraint. The error doesn't happen in master

Make sure you have a MySQL server installed and edit the settings in myproject/myproject/settings.py:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': 'test',
        'USER': 'test',
        'PASSWORD': 'test',
        'HOST': 'localhost',
        'PORT': '3306',
        'OPTIONS': {
            'init_command': 'SET default_storage_engine=InnoDB',
        },
    }
}

Then install tox and run tox to run the test.