Django Migration Data Transformation Analysis

Your understanding of the migration process is not entirely correct. Let me clarify how Django handles this situation and suggest a better approach.

Current Migration Issue

In your current migration:

1. You're updating records with membership_type values of SPONSOR (2) and MANAGING (3) to have the value CONTRIBUTING (4).
2. Then you're altering the field to have new choices where CONTRIBUTING is now represented by the value 2.

The problem: Django's AlterField operation doesn't automatically remap the stored integer values. After your migration runs, you'll have:

• Records previously set to CONTRIBUTING (4) will still have value 4
• Records you updated from SPONSOR/MANAGING will have value 4
• But your model now interprets value 2 as CONTRIBUTING and has no interpretation for value 4

Correct Approach

You need to update all records to use the new value mapping before changing the field definition. Here's how to fix it:

PYTHONCopied
def update_membership_levels(apps, schema_editor):
    # Old constants
    OLD_SPONSOR = 2
    OLD_MANAGING = 3
    OLD_CONTRIBUTING = 4
    OLD_FELLOW = 5
    
    # New constants
    NEW_CONTRIBUTING = 2
    NEW_FELLOW = 3
    
    Membership = apps.get_model('users', 'Membership')
    
    # Update SPONSOR and MANAGING to the new CONTRIBUTING value
    Membership.objects.filter(
        membership_type__in=[OLD_SPONSOR, OLD_MANAGING]
    ).update(membership_type=NEW_CONTRIBUTING)
    
    # Update old CONTRIBUTING to new CONTRIBUTING value
    Membership.objects.filter(
        membership_type=OLD_CONTRIBUTING
    ).update(membership_type=NEW_CONTRIBUTING)
    
    # Update FELLOW to new value
    Membership.objects.filter(
        membership_type=OLD_FELLOW
    ).update(membership_type=NEW_FELLOW)

def reverse_membership_levels(apps, schema_editor):
    # This is a reverse migration function if needed
    pass

class Migration(migrations.Migration):
    dependencies = [
        ('users', '0015_alter_user_first_name'),
    ]

    operations = [
        migrations.RunPython(
            update_membership_levels,
            reverse_membership_levels
        ),
        migrations.AlterField(
            model_name='membership',
            name='membership_type',
            field=models.IntegerField(choices=[(0, 'Basic Member'), (1, 'Supporting Member'), (2, 'Contributing Member'), (3, 'Fellow')], default=0),
        ),
    ]

Best Practices

This approach is actually a common pattern for handling changes to choice fields in Django:

1. First, update all data to use the new values
2. Then, update the field definition

A few additional recommendations:

1. Add a reverse migration function: It's good practice to provide a reverse function for RunPython operations, even if it's just a placeholder.

2. Consider using string choices: For future-proofing, consider using string identifiers instead of integers for choices, which makes migrations like this easier.

3. Test thoroughly: Always test migrations on a copy of production data before deploying.

4. Consider data validation: You might want to add a check after the migration to ensure no records have invalid values.

This approach ensures that all your data will be correctly mapped to the new choice values, preventing any data inconsistencies after the migration is applied.