import CoreData

let model = NSManagedObjectModel()
let entity = NSEntityDescription()
entity.name = "Entity"
model.entities = [entity]

let coordinator = NSPersistentStoreCoordinator(managedObjectModel: model)
let storeURL = URL.temporaryDirectory.appending(component: UUID().uuidString).appendingPathExtension("sqlite")
let description = NSPersistentStoreDescription(url: storeURL)
// 안해도 됨
description.setOption(true as NSNumber, forKey: NSPersistentHistoryTrackingKey)
description.shouldAddStoreAsynchronously = false
coordinator.addPersistentStore(with: description) { _, error in
  assert(error == nil)
}

let context = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
context.persistentStoreCoordinator = coordinator

func createObjects(count: Int) {
  for _ in 0..<count {
    _ = NSManagedObject(entity: entity, insertInto: context)
  }
  try! context.save()
}

let flag = false
if flag {
  try context.setQueryGenerationFrom(.current)
  createObjects(count: 10)
  let count = try! context.count(for: NSFetchRequest(entityName: entity.name!))
  assert(count == 0)
} else {
  try context.setQueryGenerationFrom(.current)
  try context.save()
  let query = context.queryGenerationToken!
  createObjects(count: 10)
  try context.setQueryGenerationFrom(query)
  let count = try! context.count(for: NSFetchRequest(entityName: entity.name!))
  assert(count == 0)
}