How to get ObjectID and search for specific ObjectID in CoreData in Swift 5? - ios

Question

I am currently working on a project with a multi user system. The user is able to create new profiles which are saved persistently using CoreData.
My problem is: Only one profile can be the active one at a single time, so I would like to get the ObjectID of the created profile and save it to UserDefaults.
Further I was thinking that as soon as I need the data of the active profile, I can simply get the ObjectID from UserDefaults and execute a READ - Request which only gives me back the result with that specific ObjectID.
My code so far for SAVING THE DATA:
// 1. Create new profile entry to the context.
let newProfile = Profiles(context: context)
newProfile.idProfileImage = idProfileImage
newProfile.timeCreated = Date()
newProfile.gender = gender
newProfile.name = name
newProfile.age = age
newProfile.weight = weight
// 2. Save the Object ID to User Defaults for "activeUser".
// ???????????????????
// ???????????????????
// 3. Try to save the new profile by saving the context to the persistent container.
do {
try context.save()
} catch {
print("Error saving context \(error)")
}
My code so far for READING THE DATA
// 1. Creates an request that is just pulling all the data.
let request: NSFetchRequest<Profiles> = Profiles.fetchRequest()
// 2. Try to fetch the request, can throw an error.
do {
let result = try context.fetch(request)
} catch {
print("Error reading data \(error)")
}
As you can see, I haven't been able to implement Part 2 of the first code block. The new profile gets saved but the ObjectID isn't saved to UserDefaults.
Also Party 1 of the second code block is not the final goal. The request just gives you back all the data of that entity, not only the one with the ObjectID I stored in User Defaults.
I hope you guys have an idea on how to solve this problem.
Thanks for your help in advance guys!

Answer 1

Since NSManagedObjectID does not conform to one of the types handled by UserDefaults, you'll have to use another way to represent the object id. Luckily, NSManagedObjectID has a uriRepresentation() that returns a URL, which can be stored in UserDefaults.
Assuming you are using a NSPersistentContainer, here's an extension that will handle the storage and retrieval of a active user Profile:
extension NSPersistentContainer {
private var managedObjectIDKey: String {
return "ActiveUserObjectID"
}
var activeUser: Profile? {
get {
guard let url = UserDefaults.standard.url(forKey: managedObjectIDKey) else {
return nil
}
guard let managedObjectID = persistentStoreCoordinator.managedObjectID(forURIRepresentation: url) else {
return nil
}
return viewContext.object(with: managedObjectID) as? Profile
}
set {
guard let newValue = newValue else {
UserDefaults.standard.removeObject(forKey: managedObjectIDKey)
return
}
UserDefaults.standard.set(newValue.objectID.uriRepresentation(), forKey: managedObjectIDKey)
}
}
}
This uses a method on NSPersistentStoreCoordinator to construct a NSManagedObjectID from a URI representation.