How to set initial value for read only field

I have a django application which is using django.contrib.admin for administrative tasks.

For one model I now need to add a field which indicates which part of the code each row was created from. I am using readonly_fields to prevent this value from being changed through the administration interface.

A default value in this field will tell me that the row was either

• created before the field was introduced
• created by code which has not been updated to set the field
• created through the administration interface

But I need better granularity than that. In particular I want to be able to distinguish between a row created by code which doesn't know about the field, and a row created through the administration interface.

Is there some way my ModelAdmin class can specify an initial value for a field mentioned in readonly_fields?

I found this solution, which appears to work:

class Admin(ModelAdmin):
    readonly_fields = ('created_by',)
    def save_form(self, request, form, change):
        r = super(Admin, self).save_form(request, form, change)
        if not change:
            assert r.created_by == CREATED_BY_UNKNOWN
            r.created_by = CREATED_BY_ADMIN
        return r

CREATED_BY_UNKNOWN and CREATED_BY_ADMIN are values defined elsewhere in my code.