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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