Add a OnetoOne field in an already populated Django ORM

Question

I have this customer model where I want to introduce a user field where the user field is
user = models.OneToOneField(User, on_delete=models.CASCADE)

but there are already some objects in this model so if i create this one to one field then it would ask me for a default value for those objects but I cannot pass default values because it'll clash with the logic.

Old model:

class Customer(models.Model):
    first_name = models.CharField(max_length=50, default=0)
    last_name = models.CharField(max_length=50, default=0)
    customer_display_name = models.CharField(max_length=50, default=0)
    customer_name = models.CharField(max_length=50, default=0)
    customer_phone = models.CharField(max_length=50, default=0)
    customer_website = models.CharField(max_length=50, default=0)
    customer_email = models.EmailField(max_length=250, default=0, blank=True, null=True)
    shipping_address = models.ManyToManyField(CustomerShippingAddress)
    billing_address = models.ManyToManyField(CustomerBillingAddress)
    active = models.BooleanField(default=True)
    remarks = models.CharField(max_length=500, default=0, blank=True, null=True)
    owner = models.ForeignKey(User, on_delete=models.CASCADE)

New model:

class Customer(models.Model):
    first_name = models.CharField(max_length=50, default=0)
    last_name = models.CharField(max_length=50, default=0)
    customer_display_name = models.CharField(max_length=50, default=0)
    customer_name = models.CharField(max_length=50, default=0)
    customer_phone = models.CharField(max_length=50, default=0)
    customer_website = models.CharField(max_length=50, default=0)
    customer_email = models.EmailField(max_length=250, default=0, blank=True, null=True)
    shipping_address = models.ManyToManyField(CustomerShippingAddress)
    billing_address = models.ManyToManyField(CustomerBillingAddress)
    active = models.BooleanField(default=True)
    remarks = models.CharField(max_length=500, default=0, blank=True, null=True)
    owner = models.ForeignKey(User, on_delete=models.CASCADE)       #Change1
    user = models.OneToOneField(User, on_delete=models.CASCADE)       #Change2
    company = models.ForeignKey(Company, on_delete=models.CASCADE)       #Change3

Reason

owner - to track who created this Customer
user - to link the Customer to User model
company - to link the company to Customer, although this connection can be find through the user who created this company

is it the right way to approach?

Answer 1

Your logic is fine, however adding non-nullable fields with django is always a bit of a pain. Here is how I do it:

1. Add the fields as nullable and create a new migration:

# models.py 
class Customer(models.Model):
    owner = models.ForeignKey(User, on_delete=models.CASCADE)                             # No changes
    user = models.OneToOneField(User, on_delete=models.CASCADE, null=True, blank=True)    # Nullable
    company = models.ForeignKey(Company, on_delete=models.CASCADE, null=True, blank=True) # Nullable

2. Generate an empty migration:

python manage.py makemigrations your_user_app --empty

3. Modify the migration to set all values:

from django.db import migrations, models

def fill_user_company(apps, schema_editor):
    Customer = apps.get_model('your_user_app', 'Customer')
    User = apps.get_model('your_user_app', 'User')
    for customer in Customer.objects.all():
        customer.user = User.objects.get(email=customer.customer_email)  # Make your own match logic here
        customer.company = customer.user.company
        customer.save()


class Migration(migrations.Migration):

    dependencies = [
        ('your_user_app', 'your_previous_migration_that_created_the_fields'),
    ]

    operations = [
        migrations.RunPython(fill_user_company),
        migrations.AlterField(
            model_name='customer',
            name='user',
            field=models.OneToOneField(
                to='auth.User',
                on_delete=models.CASCADE,
            ),
        ),
        migrations.AlterField(
            model_name='customer',
            name='company',
            field=models.ForeignKey(
                to='your_company_app.Company',
                on_delete=models.CASCADE,
            ),
        ),
    ]

4. Change your model to set the fields as non-nullable:

# models.py 
class Customer(models.Model):
    owner = models.ForeignKey(User, on_delete=models.CASCADE)      # No changes
    user = models.OneToOneField(User, on_delete=models.CASCADE)    # Non-nullable
    company = models.ForeignKey(Company, on_delete=models.CASCADE) # Non-nullable

Finally you can apply your migrations, and your new fields will be automatically created and populated.
Note that you can merge your 2 migrations file in a single one if you prefer.