 
         fileprivate init(
             keyPath: KeyPathString,
-            isOptional: Bool,
             customGetter: ((_ object: ObjectProxy<O>, _ field: ObjectProxy<O>.FieldProxy<V>) -> V)?,
             customSetter: ((_ object: ObjectProxy<O>, _ field: ObjectProxy<O>.FieldProxy<V>, _ newValue: V) -> Void)? ,
             affectedByKeyPaths: @escaping () -> Set<KeyPathString>) {

            self.entityDescriptionValues = {
                (
                    attributeType: .undefinedAttributeType,
                    isOptional: isOptional,
                    isTransient: true,
                    allowsExternalBinaryDataStorage: false,
                    versionHashModifier: nil,


        self.init(
            keyPath: keyPath,
            customGetter: customGetter,
            customSetter: customSetter,
            affectedByKeyPaths: affectedByKeyPaths
