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main.swift

// To run this code:
// 1. Create a macOS playground from Xcode.
// 2. Copy below code into the playground.
// 3. Click the Run button to see the console output.
import Foundation
import CoreData

func createObjectModel() -> NSManagedObjectModel {
    let nameAttr = NSAttributeDescription()
    nameAttr.name = "username"
    nameAttr.type = .string
    let nameProp = NSPropertyDescription()
    nameProp.name = "username"
    let user = NSEntityDescription()
    user.name = "User"
    user.properties = [nameAttr]
    let model = NSManagedObjectModel()
    model.entities = [
        user
    ]
    return model
}

let container = NSPersistentContainer(name: "Test", managedObjectModel: createObjectModel())
container.persistentStoreDescriptions = [
    NSPersistentStoreDescription(url: URL(filePath: "/dev/null"))
]
container.loadPersistentStores { _, _ in }

let viewContext = container.viewContext
viewContext.automaticallyMergesChangesFromParent = true

let user = NSEntityDescription.insertNewObject(forEntityName: "User", into: viewContext)
user.setValue("Foo", forKey: "username")
try viewContext.save()

let background = container.newBackgroundContext()
try background.performAndWait {
    let user = try background.existingObject(with: user.objectID)
    user.setValue("Bar", forKey: "username")
    try background.save()
}

print(user.value(forKey: "username"))

DispatchQueue.main.async {
    print(user.value(forKey: "username"))
}