Writing a Django migration that adds a unique field based off another field

Question

I have a model SessionCategory with a unique field name. Certain pivotal instances of this model are referenced by name; the only problem is that name is also editable in our internal dashboard so that a user could inadvertently 'break' these references by changing the name.

To solve this, I'd like to create a new field name_slug which is a slugified version of name, also with a unique constraint. I've tried to do the following:

from django.db import models
from django.utils.text import slugify
from django.db.models.signals import pre_save
from django.dispatch import receiver



class SessionCategory(models.Model):
    name = models.CharField(max_length=255, unique=True)
    name_slug = models.CharField(max_length=255, unique=True)


@receiver(pre_save, sender=SessionCategory)
def create_name_slug(sender, instance, **kwargs):
    if not instance.name_slug:
        instance.name_slug = slugify(instance.name)

where I've added the name_slug unique field. The problem is that if I try to python manage.py makemigrations, I get the following prompt:

(venv) Kurts-MacBook-Pro-2:lucy-web kurtpeek$ python manage.py makemigrations
You are trying to add a non-nullable field 'name_slug' to sessioncategory without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Quit, and let me add a default in models.py
Select an option: 

I've come across a workflow at https://docs.djangoproject.com/en/dev/howto/writing-migrations/#migrations-that-add-unique-fields for writing migrations that create unique fields. In that example, the unique field is generated using the 'general' function uuid.uuid4(). In my case, however, I need access to the name of the specific instance, like I was hoping to obtain by connecting to the pre_save signal.

How can I create this migration and maintain the unique constraint?

Answer 1

I think the RunPython feature will help you.

Step1. You have already a migration file before adding name_slug field. Then create a new migration file with, name_slug = models.CharField(max_length=255, unique=True,null=True).The file will be like this

app_name/migrations/0003_some_name.py

from __future__ import unicode_literals

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('sample', '0006_sessioncategory'),
    ]

    operations = [
        migrations.AddField(
            model_name='sessioncategory',
            name='name_slug',
            field=models.CharField(max_length=255, null=True, unique=True),
        ),
    ]

**Step2.** Add a custom migration scrip by using `RunPython` as below,

from __future__ import unicode_literals
from django.db import migrations, models
from django.utils.text import slugify
class Migration(migrations.Migration):
def forwards_func(apps, schema_editor):
SessionCategory = apps.get_model("sample", "SessionCategory")
for instance in SessionCategory.objects.all():
if not instance.name_slug:
instance.name_slug = slugify(instance.name)
instance.save()
def reverse_func(apps, schema_editor):
    pass</b>

dependencies = [
    ('sample', '0006_sessioncategory'),
]

operations = [
    migrations.AddField(
        model_name='sessioncategory',
        name='name_slug',
        field=models.CharField(max_length=255, null=True, unique=True),
    ),
    <b>migrations.RunPython(forwards_func, reverse_func

                         )</b>
]</code></pre>

Step 3. do the migration by python manage.py migrate
That's it !



Warning!
do not run python manage.py migrate command on step 1 (only makemigrations command)

Note
The migrations files are generated in my local system to reproduce the behaviour.It maybe different for you