Removing composed indexes fails on MySQL

[Sergey Fursov]

Removing composed indexes that are being created using UniqueConstraint or Index added to Meta.contraints/Meta.indexes respectively fails on MySQL due to implicit optimization made by MySQL with omitting indexes for a foreign key that were initially included in the index.

The bug might be considered as a regression for the previously fixed #24757

The initial state of example models

from django.db import models

class ModelA(models.Model):
    pass

class ModelB(models.Model):
    a = models.ForeignKey(MyModelA)
    b = models.IntegerField()

    class Meta:
        constraints = models.UniqueConstraint(fields=['a', 'b'], name='modelb_uniq')
        indexes = models.Index(fields=['a', 'b'], name='modelb_index')

the final state:

from django.db import models

class ModelA(models.Model):
    pass

class ModelB(models.Model):
    a = models.ForeignKey(MyModelA)
    b = models.IntegerField()

considering the example states of the models above, MySQL first optimizes schema by omitting index for ModelB.a FK field as it is already included in composed index, and then during removing the constraint or the index migration fails with

Cannot drop index 'modelb_uniq': needed in a foreign key constraint"

or

Cannot drop index 'modelb_index': needed in a foreign key constraint"

[Mariusz Felisiak]

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