exclude changed to readonly_fileds in django1.2

django-1.1 to 1.2 exclude changed to readonly_fields

http://ygamretuta.me/2011/02/17/django-1-2-exclude-does-not-work-on-datetimefield-with-auto_now-and-auto_now_add/

class ProductAdmin(admin.ModelAdmin):
    list_display = ('name', 'description', 'created_at', 'updated_at',)
    list_display_links = ('name',)
    list_per_page = 50
    ordering = ['-created_at']
    search_fields = ['name', 'description']
    exclude = ('created_at', 'updated_at',)

class ProductAdmin(admin.ModelAdmin):
    list_display = ('name', 'description', 'created_at', 'updated_at',)
    list_display_links = ('name',)
    list_per_page = 50
    ordering = ['-created_at']
    search_fields = ['name', 'description']
    readonly_fields = ('created_at', 'updated_at',)