Re: Migrations force me to have, forever, any name/reference I use in field validators=, choices=, whatever, valid forever (even if the requirements change).

Yes, it's expected behavior. Please see the documentation on the topic:
https://docs.djangoproject.com/en/stable/topics/migrations/#historical-models

On Wednesday, February 25, 2015 at 3:25:19 PM UTC-5, Luis Masuelli wrote:

I have an issue with migrations.

Suppose I declare (in my application, with name myapp) a field with a validators= declared. In such value (which is an iterable), I declare a function:

class MyModel(models.Model):
    """
    This one is identifiable.
    """

    identifier = models.CharField(max_length=10, unique=True, validators=[valid_identifier])

Assume I created the migration with makemigrations:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations
from django.conf import settings
import django.core.validators
import micnt.models


class Migration(migrations.Migration):

    dependencies = [
        ...
    ]

    operations = [
        migrations.CreateModel(
            name='MyModel',
            fields=[
                ('identifier', models.CharField(max_length=10, unique=True, validators=[myapp.models.valid_identifier])),
            ],
            options={
            },
            bases=(models.Model,),
        ),
    ]

Assume valid_identifier is any external callable which will take one positional argument, and eventually raise ValidationError under certain conditions.

Now the requirements vary: The identifier may be national or foreign, so the country is required as an additional field, and also a discriminator field. Additionally, the validation I must perform is cross-field, so instead of using something in validators, I define a clean() method, and make the combination of country/identifier unique, instead of just the identifier unique:

class MyModel(models.Model):
    """
    This one is identifiable.
    """

    identifier_type = models.CharField(max_length=10, choices=(('passport', _(u'Passport')), ('national', _(u'National'))), null=False)
    identifier = models.CharField(max_length=10)
    country = models.ForeignKey(Country, null=False)  # assume Country model exists

    class Meta:
        unique_together = (('identifier', 'country'),)

    def clean(self):
        raise ValidationError('Implement this!')
   

And then, I don't need anymore the valid_identifier function, since my code does not use it anymore. So I delete it.

Now I will create the migration with makemigrations as usual. What will happen?

a. The command will succeed.
b. The command will explode (i. e. a normal Python exception).
c. The command will create an inconsistent migration, without exploding.
d. Jesus will come again.
e. The command will get into a kind of endless loop.

If you guesses "b", you were right. Django will report an error, since myapp.models.valid_identifier does not exist anymore (AttributeError).

So, once I set a value for validators= parameter and create a migration, I am forced to keep such reference valid for the rest of my life (e.g. by having valid_identifier=None, if I don't need the function anymore), or else the migrations will not work again since they will be traversed, imported, and will get such AttributeError.

Is this an expected, normal, behavior?

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