Django unique constraint not working properly [duplicate] - mysql

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Unique constraint that allows empty values in MySQL
(3 answers)
Closed 2 years ago.
I have a model with a custom _id that has to be unique, and soft delete, deleted objects don't have to have a unique _id, so I did it as follows:
class MyModel(models.Model):
_id = models.CharField(max_length=255, db_index=True)
event_code = models.CharField(max_length=1, blank=True, default='I')
deleted = models.BooleanField(default=False)
deleted_id = models.IntegerField(blank=True, null=True)
objects = MyModelManager() # manager that filters out deleted objects
all_objects = MyModelBaseManager() # manager that returns every object, including deleted ones
class Meta:
constraints = [
UniqueConstraint(fields=['_id', 'event_code', 'deleted', 'deleted_id'], name='unique_id')
]
def delete(self, *args, **kwargs):
self.deleted = True
self.deleted_id = self.max_related_deleted_id() + 1
self.save()
def undelete(self, *args, **kwargs):
self.deleted = False
self.deleted_id = None
self.save()
def max_related_deleted_id(self):
# Get max deleted_id of deleted objects with the same _id
max_deleted_id = MyModel.all_objects.filter(Q(_id=self._id) & ~Q(pk=self.pk) & Q(deleted=True)).aggregate(Max('deleted_id'))['deleted_id__max']
return max_deleted_id if max_deleted_id is not None else 0
The whole logic of the deleted_id is working, I tested it out, the problem is, the UniqueConstraint is not working, for example:
$ MyModel.objects.create(_id='A', event_code='A')
$ MyModel.objects.create(_id='A', event_code='A')
$ MyModel.objects.create(_id='A', event_code='A')
$ MyModel.objects.filter(_id='A').values('pk', '_id', 'event_code', 'deleted', 'deleted_id')
[{'_id': 'A',
'deleted': False,
'deleted_id': None,
'event_code': 'A',
'pk': 1},
{'_id': 'A',
'deleted': False,
'deleted_id': None,
'event_code': 'A',
'pk': 2},
{'_id': 'A',
'deleted': False,
'deleted_id': None,
'event_code': 'A',
'pk': 3}]
Here is the migration that created the unique constraint:
$ python manage.py sqlmigrate myapp 0003
BEGIN;
--
-- Create constraint unique_id on model MyModel
--
ALTER TABLE `myapp_mymodel` ADD CONSTRAINT `unique_id` UNIQUE (`_id`, `event_code`, `deleted`, `deleted_id`);
COMMIT;
Any help is appreciated!
Django version = 2.2
Python version = 3.7
Database = MySQL 5.7

Answer 1

Ok, I figured out my problem, I'm gonna post this here in case someone runs into it:
The problem is with MySQL, as stated in this post, mysql allows multiple null values in a unique constraint, so I had to change the default of deleted_id to 0 and now it works.