Using Dynamic Settings in Django GeneratedField

When working with Django's GeneratedField, you're encountering a common issue where the value of settings.SOME_HOST is being evaluated and "frozen" at migration creation time rather than being dynamically evaluated when the migration is applied.

This happens because Django's migration system serializes the expression, capturing the actual value of the setting rather than maintaining a reference to the setting itself.

Solution: Custom Expression Class

To solve this problem, you can create a custom expression class that will evaluate the setting at runtime:

PYTHONCopied
from django.db.models import Func, Value, CharField
from django.conf import settings

class SettingValue(Func):
    function = ''
    template = '%(expressions)s'
    
    def __init__(self, setting_name, **kwargs):
        self.setting_name = setting_name
        super().__init__(**kwargs)
    
    def as_sql(self, compiler, connection, **extra_context):
        # Get the current value of the setting at runtime
        setting_value = getattr(settings, self.setting_name)
        return Value(setting_value).as_sql(compiler, connection, **extra_context)
    
    def deconstruct(self):
        path, args, kwargs = super().deconstruct()
        kwargs['setting_name'] = self.setting_name
        return path, args, kwargs

Then modify your model to use this custom expression:

PYTHONCopied
from django.db import models
from django.db.models.functions import Concat

class Foo(models.Model):
    name = models.CharField(max_length=30)
    link = models.GeneratedField(
        expression=Concat(
            SettingValue('SOME_HOST'),
            models.Value("/api/v2/"),
            "name"
        ),
        output_field=models.URLField(),
        db_persist=False,
    )

With this approach, the SettingValue expression will be serialized in the migration, but the actual value of settings.SOME_HOST will be retrieved at the time the migration is applied or when the database expression is evaluated.

This ensures that if you change SOME_HOST in your settings, the generated field will use the updated value without requiring a new migration.

Note that this approach works best with db_persist=False as you've already specified, since that ensures the expression is evaluated each time the field is accessed rather than being stored in the database.