migration to UUID id field doesn't properly convert integers (SQLite).

[Martin Baker]

This may be present in other DBs, but I've noticed it with SQLite under Django 2.2.1.

Steps to reproduce:

1. Start a model with an implicit integer id, and create and run the migration:

from django.db import models

class Something(models.Model):
    name = models.CharField(max_length=32)

2. Then create one instance via the shell (python manage.py shell):

from uuidbug.models import Something
s = Something(name="One")
s.save()

In the DB it looks like this (can use python manage.py dbshell):

sqlite> .headers on
sqlite> select * from uuiditem_something;
name|id
One|1

3. Edit the model to use UUIDs:

import uuid

from django.db import models

class Something(models.Model):
    name = models.CharField(max_length=32)
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)

4. After creating and running the migration, add another instance:

from uuidbug.models import Something
s = Something(name="After UUID-ization")
s.save()

DB now looks like this:

sqlite> select * from uuidbug_something;
name|id
One|1
After UUID-ization|693e1942d7d142289bb9fb3664f2c11a

Note that the id for the first instance has not been converted to a UUID properly. If you try to access the objects, you get an error:

>>> Something.objects.all()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/query.py", line 653, in first
    for obj in (self if self.ordered else self.order_by('pk'))[:1]:
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/query.py", line 274, in __iter__
    self._fetch_all()
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/query.py", line 1242, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/query.py", line 72, in __iter__
    for row in compiler.results_iter(results):
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1044, in apply_converters
    value = converter(value, expression, connection)
  File "/Users/martin/Connery/opencc-backend/venv/lib/python3.7/site-packages/django/db/backends/sqlite3/operations.py", line 294, in convert_uuidfield_value
    value = uuid.UUID(value)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/uuid.py", line 160, in __init__
    raise ValueError('badly formed hexadecimal UUID string')
ValueError: badly formed hexadecimal UUID string

[Mariusz Felisiak]

Thanks for this report, however changing column data type is always a sensitive operation and you need to be aware of database caveats. SQLite doesn't have a native UUID data type, so Django handles it as a char(32). Altering data type from integer to char is possible with auto-conversion that's why you didn't get any issue when running this migration. I don't think that there is much that Django can do here.

To fix existing data you can add ​custom data migration and use Python's uuid.UUID(int=existing_pk) to convert data from int to UUID.

[Martin Baker]

I guess to me this just seems like something Django should be able to automagically take care of. This seems like a semi-frequent use case: I made some objects, oh oops I want them to be non-sequential / UUID-y.