How Can I Tell If My NSManagedObject Resides in a Read Only NSPersistentStore? - cocoa

Question

I would like to add read-only example/tutorial data to my Core Data based macOS app.
I will include an SQL file in my application bundle containing the example data. My NSPersistentContainer will have 2 NSPersistentStores, one writable and one read only. I will only have a default configuration for my model since both stores will have the same model.
My UI will need to know if the data displayed is read only or not, for example, to stop this data being draggable.
I know that NSManagedObject does not support a readonly state, see and : Is it possible to return NSManagedObjects as read-only in Core Data? ...and the docs.
I think the best approach would be to add a readonly property to my NSManagedObject derived class that can be queried where necessary. However, I can't see how I could easily set this property! I can't find a direct link to an NSPersistentStore from an NSManagedObject.
I could set up an NSFetchRequest and specify the read only store and see if the NSManagedObject is in it, but that seems a little ridiculous.
Am I missing something more obvious here please?

Answer 1

With thanks to pbasdf for his suggestion...
I could find no straight-forward way to achieve this. I had to move away from using NSPersistentContainer to simplify my Core Data stack. However, I think this is a fairly elegant solution if you need a small subset of your graph to be readonly.
I subclassed NSPersistentStoreCoordinator to cache the NSManagedObjectIDs of any readonly store added to it:
class GraphStoreCoordinator: NSPersistentStoreCoordinator
{
override init(managedObjectModel model: NSManagedObjectModel)
{
readOnlyTestContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
super.init(managedObjectModel: model)
readOnlyTestContext.persistentStoreCoordinator = self
NotificationCenter.default
.addObserver(forName: .NSPersistentStoreCoordinatorStoresDidChange,
object: self, queue: nil) { [unowned self] notification in
// userInfo will be in this form for add/remove keys - not supporting migration here
guard let userInfo = notification.userInfo as? [String: [NSPersistentStore]] else {
unhandledError("Invalid userInfo for NSPersistentStoreCoordinatorStoresDidChange.") }
userInfo[NSAddedPersistentStoresKey]?.forEach { self.didAddStore($0) }
userInfo[NSRemovedPersistentStoresKey]?.forEach { self.didRemoveStore($0) }
}
}
deinit {
NotificationCenter.default
.removeObserver(self, name: .NSPersistentStoreCoordinatorStoresDidChange, object: self)
}
private func didAddStore(_ store: NSPersistentStore) {
guard store.isReadOnly else { return }
var addedObjects = Set<NSManagedObjectID>()
baseEntityNames.forEach { entityName in
let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: entityName)
fetchRequest.affectedStores = [store]
do {
let addedEntityObjects = try readOnlyTestContext.fetch(fetchRequest)
addedObjects = addedObjects.union(addedEntityObjects.map { $0.objectID })
} catch {
unhandledError("Failed to fetch all \(entityName) for read only check: \(error)") }
}
readOnlyObjects[store.identifier] = addedObjects
}
private func didRemoveStore(_ store: NSPersistentStore) {
guard store.isReadOnly else { return }
readOnlyObjects.removeValue(forKey: store.identifier)
}
/// Returns the minimum set of entities that can be fetched for readonly checking
private lazy var baseEntityNames: [String] = {
return managedObjectModel.entitiesByName.compactMap { $1.superentity == nil ? $0 : nil }
}()
private var readOnlyTestContext: NSManagedObjectContext
/// Readonly objectIDs keyed per persistent store
private var readOnlyObjects = [String : Set<NSManagedObjectID>]()
internal func isObjectReadOnly(_ objectID: NSManagedObjectID) -> Bool {
return readOnlyObjects.contains(where: { $1.contains(objectID) } )
}
}
I then added an extension to NSManagedObject to that queries its NSPersistentStoreCoordinator for read-only status:
public extension NSManagedObject
{
/// Does this managed object reside in a read-only persistent store?
var isReadOnly: Bool {
guard let coordinator = managedObjectContext?
.persistentStoreCoordinator as? GraphStoreCoordinator else {
unhandledError("Should only check readonly status in a GraphStoreCoordinator") }
return coordinator.isObjectReadOnly(objectID)
}
}