Django: convert CharField primary key to IntegerField - django

Question

I have a model (devices) with a CharField primary key containing integers that I would like to convert to an IntegerField. The problem is the devices are also used as a foreign key in another tables meaning when I change the field in my models.py and run the makemigrations/migrate I keep getting an error prompting me to drop the dependent objects first. How can I change the device primary key to an IntegerField without dropping the dependent objects?
models.py
class Device(models.Model):
id = models.CharField(primary_key=True, unique=True, max_length=255)
date_created = models.DateField()
class Station(models.Model):
id = models.AutoField(primary_key=True)
device = models.ForeignKey('Device', on_delete=models.PROTECT)
error when running the migration
constraint api_station_device_id_117642ec_fk_api_device_id on table api_station depends on index api_device_pkey
HINT: Use DROP ... CASCADE to drop the dependent objects too.

Answer 1

The error message you are receiving is due to the fact that changing the primary key type of the Device model will require changing the foreign key type of the device field in the Station model as well. This will create a dependency that requires the api_station_device_id_117642ec_fk_api_device_id constraint on the api_station table to be dropped, which is not allowed by the database without first dropping the dependent objects.
You can create a new id field of type IntegerField in the Device model and set it as the primary key. You can then populate this new field with the integer values of the old id field and remove the old id field after updating the Station model to reference the new id field. eg in your models.py file
class Device(models.Model):
id = models.AutoField(primary_key=True)
id_char = models.CharField(unique=True, max_length=255)
date_created = models.DateField()
def save(self, *args, **kwargs):
# set the integer value of the new id field to the integer value of the old id field
if self.id_char is not None:
self.id = int(self.id_char)
super().save(*args, **kwargs)
class Station(models.Model):
id = models.AutoField(primary_key=True)
device = models.ForeignKey('Device', on_delete=models.PROTECT, to_field='id')
Write a script to populate the id field with the integer values of
the id_char field for all existing Device objects.
Here are the steps to create a custom Django management command that runs the script:
Create a new Python module (e.g., update_device_ids.py) in a Django
app that has the Device model. You can put this module in the
management/commands/ directory within your app to define a new
management command.
Import the relevant Django models and the BaseCommand class from the
django.core.management.base module. Your module may look like this
from django.core.management.base import BaseCommand
from myapp.models import Device
class Command(BaseCommand):
help = 'Updates the Device id field with the integer value of the id_char field for all devices'
def handle(self, *args, **options):
devices = Device.objects.all()
for device in devices:
if device.id_char is not None:
device.id = int(device.id_char)
device.save()
Then follow the below process:
Update the id field in your Device model from CharField to
IntegerField.
Create a new migration by running python manage.py makemigrations.
Before running the migration, create a new temporary field in your
Device model that will store the integer value of the old id_char
field. For example, you could add the following line to your Device
model:
id_int = models.IntegerField(null=True, blank=True)
Run the python manage.py update_device_ids command to populate the
id_int field with the integer values of the old id_char field.
Create a data migration by running python manage.py makemigrations --empty yourappname.
Edit the data migration file (in the migrations/ directory) and add
the following code to the forwards() method:
from django.db import migrations
class Migration(migrations.Migration):
dependencies = [
('yourappname', 'the_previous_migration'),
]
operations = [
migrations.AddField(
model_name='device',
name='id_int',
field=models.IntegerField(null=True, blank=True),
),
migrations.RunSQL('UPDATE yourappname_device SET id_int = CAST(id as integer)'),
migrations.RemoveField(
model_name='device',
name='id',
),
migrations.RenameField(
model_name='device',
old_name='id_int',
new_name='id',
),
]
This code adds a new IntegerField field called id_int, populates it with the integer values of the old id_char field using a SQL command, removes the old id_char field, and renames the id_int field to id.
Finally, run the data migration by running python manage.py migrate.