Swift Core Data setting relationship Unrecognized selector send to instance in Unit Test

I have two entities 'Image' and 'Gift'. 'Gift' has a relationship to 'Image' called images which is One-To-Many. I have automatically generated the ManagedObject classes, and the class specified in the entity model is prefixed with my project name e.g. MyDataModel.Gift.

I am trying to unit test the code generated. I have created the following base class which all Core Data access will inherit from so I can test 'in-memory' the data model.

import UIKit
import XCTest
import CoreData

class CoreDataTestBase: XCTestCase {

    lazy var applicationDocumentsDirectory: NSURL = {
        // The directory the application uses to store the Core Data store file. This code uses a directory named "book.persistence.GiftReminder" in the application's documents Application Support directory.
        let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
        return urls[urls.count-1] as! NSURL
        }()

    lazy var managedObjectModel: NSManagedObjectModel = {
        // The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
        let modelURL = NSBundle.mainBundle().URLForResource("MyDataModel", withExtension: "momd")!
        return NSManagedObjectModel(contentsOfURL: modelURL)!
        }()

    lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator? = {
        // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it. This property is optional since there are legitimate error conditions that could cause the creation of the store to fail.
        // Create the coordinator and store
        var coordinator: NSPersistentStoreCoordinator? = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
        let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("MyDataModel.sqlite")
        var error: NSError? = nil
        var failureReason = "There was an error creating or loading the application's saved data."
        if coordinator!.addPersistentStoreWithType(NSInMemoryStoreType, configuration: nil, URL: url, options: nil, error: &error) == nil {
            coordinator = nil
            // Report any error we got.
            var dict = [String: AnyObject]()
            dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
            dict[NSLocalizedFailureReasonErrorKey] = failureReason
            dict[NSUnderlyingErrorKey] = error
            error = NSError(domain: "YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
            // Replace this with code to handle the error appropriately.
            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            NSLog("Unresolved error \(error), \(error!.userInfo)")
            abort()
        }

        return coordinator
        }()

    lazy var managedObjectContext: NSManagedObjectContext? = {
        // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
        let coordinator = self.persistentStoreCoordinator
        if coordinator == nil {
            return nil
        }
        var managedObjectContext = NSManagedObjectContext()
        managedObjectContext.persistentStoreCoordinator = coordinator
        return managedObjectContext
        }()

    // MARK: - Core Data Saving support

    func saveContext () {
        if let moc = self.managedObjectContext {
            var error: NSError? = nil
            if moc.hasChanges && !moc.save(&error) {
                // Replace this implementation with code to handle the error appropriately.
                // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                NSLog("Unresolved error \(error), \(error!.userInfo)")
                abort()
            }
        }
    }

    override func setUp() {
        super.setUp()
    }

    override func tearDown() {
        self.managedObjectContext = nil
        super.tearDown()
    }

}

Then I created a class which will test that I can create the entities.

import Foundation
import UIKit
import XCTest
import CoreData

class ManagedObjectsTests: CoreDataTestBase {

    /*
    Test we can save an image
    */
    func testThatWeCanCreateImage() {
        let entity = NSEntityDescription.entityForName("Image", inManagedObjectContext: managedObjectContext!)
        let image = Image(entity: entity!, insertIntoManagedObjectContext: managedObjectContext!)
        let aImage = emptyImage(CGSizeMake(1.0, 1.0))
        image.picture = UIImageJPEGRepresentation(aImage, 1)

        XCTAssertNotNil(image, "Unable to create a image")
    }

    func emptyImage(size: CGSize, scale: CGFloat = UIScreen.mainScreen().scale) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, false, scale)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }

    /*
    Test we can save a gift
    */
    func testThatWeCanCreateGift() {
        let entity = NSEntityDescription.entityForName("Gift", inManagedObjectContext: managedObjectContext!)
        let gift = Gift(entity: entity!, insertIntoManagedObjectContext: managedObjectContext!)
        gift.name = "Test"
        gift.price = 10.0
        gift.location = "London"

        XCTAssertNotNil(gift, "Unable to create a gift")
    }

    /*
    Test we can save a gift with an image
    */
    func testThatWeCanSaveGiftWithImage() {
        let entity = NSEntityDescription.entityForName("Gift", inManagedObjectContext: managedObjectContext!)
        let gift = Gift(entity: entity!, insertIntoManagedObjectContext: managedObjectContext!)
        let image = Image(entity: entity!, insertIntoManagedObjectContext: managedObjectContext!)

        XCTAssertNotNil(gift, "Unable to create gift")
        XCTAssertNotNil(image, "Unable to create image")

        let aImage = emptyImage(CGSizeMake(1.0, 1.0))
        gift.name = "Test"
        gift.price = 12.0
        gift.location = "London"

        println(NSStringFromClass(gift.classForCoder))
        image.gift = gift

        var error: NSError? = nil
        managedObjectContext?.save(&error)

        XCTAssertNil(error, "Failed to save the context with error \(error), \(error?.userInfo)")
    }

}

I created two tests 'testThatWeCanCreateImage' and 'testThatWeCanCreateGift' which test we can create the entities. These to tests pass successfully. The third test is 'testThatWeCanSaveGiftWithImage' which creates a Gift entity and an Image entity, I then create the relationship when image.gift = gift, this should work with no error. However, I get the error -[Gift setGift:]: unrecognized selector sent to instance 0x7ffb7b45dd20 What am I doing wrong? Or is this an error with Xcode?