mom + psc issue.swift

import CoreData

func newManagedObjectModel() -> NSManagedObjectModel {
  // you get new instance of MOM with the same properties
  // each time you call the function.
  // this function uses entity description returned by 
  // newEntityDescription function
}

func newEntityDescription() -> NSEntityDescription {
  // again, you get new instance of entity description
  // with same properties each time you call the function
}

final class ManagedFoo: NSManagedObject {
  @NSManaged var bar: String
}

func test_SavingWithManagedObjectContext() {
  let coordinator = NSPersistentStoreCoordinator(managedObjectModel: newManagedObjectModel())
  let context = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
  context.persistentStoreCoordinator = coordinator
  
  let foo = ManagedFoo(entity: newEntityDescription(), insertIntoManagedObjectContext: nil)
  context.insert(foo)
  try! context.save() // code will crash here with pretty obscure error 134020 “Store cannot hold instances of entity”
}

One solution:

let managedObjectModel: NSManagedObjectModel = {
  // you get new instance of MOM with the same properties
  // each time you call the function.
  // this function uses entity description returned by 
  // newEntityDescription function
}()

let newEntityDescription: NSEntityDescription = {
  // again, you get new instance of entity description
  // with same properties each time you call the function
}()