UniqueConstraint with ForeignKey cannot be removed on MySQL/MariaDB

[Christopher Currie]

Migrations containing migrations.AddConstraint(model_name='xxx', constraint=models.UniqueConstraint(fields=[], name='yyy')) will fail when unapplied on MySQL/MariaDB backends.

Given the database engine is mysql and the following models.py:

from django.db import models


class Parent(models.Model):
    name = models.CharField(max_length=30)


class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)
    name = models.CharField(max_length=30)

And the 2 following migrations:

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('data', '0001_initial'),
    ]

    operations = [
        migrations.AddConstraint(
            model_name='child',
            constraint=models.UniqueConstraint(fields=('parent', 'name'), name='unique_parent_child_name'),
        ),
    ]

class Migration(migrations.Migration):

    dependencies = [
        ('data', '0002_auto_20200627_1823'),
    ]

    operations = [
        migrations.RemoveConstraint(
            model_name='child',
            name='unique_parent_child_name',
        ),
    ]

Running ./manage.py migrate produces the following error:

Operations to perform:
  Apply all migrations: admin, auth, contenttypes, data, sessions
Running migrations:
  Applying data.0003_auto_20200627_1838...Traceback (most recent call last):
  [...]
MySQLdb._exceptions.OperationalError: (1553, "Cannot drop index 'unique_parent_child_name': needed in a foreign key constraint")

The same migrations work against sqlite. The error reproduces against mariadb 10, mysql 8, and mysql 5, with Django versions 2.2 and 3.0.

This issue was previously reported for Django 1.8:

https://code.djangoproject.com/ticket/24757

That issue was resolved against the deprecated unique_together form. Using unique_together in place of UniqueConstraint works as expected.

A complete project reproducing this issue is attached.

[Christopher Currie]

Project reproducing error

[Mariusz Felisiak]

Thanks! IMO we can close this as a duplicate of #31335 (see Simon's comments).