#34332: Migrations for fields with model-referencing defaults break later
-------------------------------------+-------------------------------------
Reporter: Kenny | Owner: nobody
Loveall |
Type: Bug | Status: new
Component: Database | Version: 3.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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**Use case/Goal:** I want to generate short (4-8 character), unique codes
for orders on a website (think confirmation numbers of airlines, car
license plates or the like). With sufficient length, I could just UUIDs
and be reasonably guaranteed that I won't see a collision in my lifetime,
with the smaller dataspace to pick from, this becomes no longer true.
Therefore, I generate a new random one, check to see if it's already in
use, and if so, try another one until I find one that works (and log a
warning on a first collision and error out if I try too many).
**Attempted code:**
models.py:
{{{
def generate_order_public_reference_number():
attempts = 0
while True:
candidate_str = 'o' + uuid.uuid4().hex[:5]
try:
Order.objects.get(public_reference_number=candidate_str)
except Order.DoesNotExist:
return candidate_str
attempts += 1
if attempts > 10:
raise RuntimeError("Unable to find an unused
public_reference_number after 10 attempts.")
class Order(models.Model):
...
public_reference_number = models.CharField(max_length=10,
null=False,
default=generate_order_public_reference_number)
}}}
migration file:
{{{
def forwards(apps, schema_editor):
Order = apps.get_model('orders', 'Order')
for o in Order.objects.all():
while True:
candidate_str = 'o' + uuid.uuid4().hex[:6]
try:
Order.objects.get(public_reference_number=candidate_str)
except Order.DoesNotExist:
o.public_reference_number = candidate_str
o.save()
break
class Migration(migrations.Migration):
dependencies = [
('orders', '0005_auto_20221120_0218'),
]
operations = [
migrations.AddField(
model_name='order',
name='public_reference_number',
field=models.CharField(null=True, default=None,
max_length=10),
),
migrations.RunPython(forwards, migrations.RunPython.noop),
migrations.AlterField(
model_name='order',
name='public_reference_number',
field=models.CharField(default=orders.models.generate_order_public_reference_number,
max_length=10), ),
]
}}}
**What happens:** After adding a new field in the future to Order, I get
an exception when attempting to apply this migration because the migration
engine calls the default callable in the migration to get the default to
set on the rows even though I have guaranteed in the row above that there
are no rows that will need that default.
**What I expect to happen:** Only try to call the default function if
there's a need for the default value (or even the ''potential'' for a
need). In this case, the default was already set on the model so we know
for sure there's no rows that will need a default filled in.
--
Ticket URL: <https://code.djangoproject.com/ticket/34332>
Django <https://code.djangoproject.com/>
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