managedObjectContext cannot be used more than once?

Question

I have an app which uses CoreData to save different playlists using MPMediaPicker. After saving the first one, while I try to add the second one, the app crashes. Below is the entire debug queue.

Creation of the first record.

musicTemp = <MPMediaItemCollection: 0x13df04440>=========>prepareForSegue
saveValues()
▿ Optional<NSManagedObjectContext>
  - Some : <NSManagedObjectContext: 0x13de88a60>

musicTemp = <MPMediaItemCollection: 0x13df04440>=========>saveValues
ModelName.Rule
Optional(<NSManagedObjectContext: 0x13de88a60>)
Number of rows = 1
Number of rows = 1
musicTemp = <MPMediaItemCollection: 0x13df04440>=========>RuleViewCell
mainManagedObjectContext.save

End of first record creation. Start of second record creation,

musicTemp = <MPMediaItemCollection: 0x13dea7620>=========>prepareForSegue
saveValues()
▿ Optional<NSManagedObjectContext>
  - Some : <NSManagedObjectContext: 0x13de88a60>

musicTemp = <MPMediaItemCollection: 0x13dea7620>=========>saveValues
ModelName.Rule
Optional(<NSManagedObjectContext: 0x13de88a60>)
2016-09-10 11:39:02.469 AppName[5237:2139350] -[MPMediaItemCollection compare:]: unrecognized selector sent to instance 0x13df04440
All Exceptions
error: use of undeclared identifier 'mainManagedObjectContext'
error: 1 errors parsing expression 

App Crashes!!

The related code which does the addition of records,

// Method to save the extracted values to CoreData Objects
private func saveValues() {

    // Initialize entity description
    let RuleDescription = NSEntityDescription.entityForName("Rule",inManagedObjectContext:mainManagedObjectContext!)
    let rule = Rule(entity: RuleDescription!, insertIntoManagedObjectContext: mainManagedObjectContext)
    // Set object values from temporary variables
    rule.music = musicTemp!
    print("musicTemp = \(musicTemp!.count)=========>saveValues")
    // Save the object in privateManagedObjectContext
    print(NSStringFromClass(rule.classForCoder))
    print(mainManagedObjectContext)
    if ((mainManagedObjectContext?.hasChanges) != nil) {
        do {
            try mainManagedObjectContext?.save()
            print("mainManagedObjectContext.save")
        } catch let saveError as NSError {
            print("(AddRuleViewController)(mainManagedObjectContext)saveError: \(saveError), \(saveError.userInfo)")
        }
    }
}

mainManagedObjectContext is referenced using dependency injection. I am not able to figure out why it is able to save the first time, but not again. Actually, it complains about mainManagedObjectContext.

Answer 1

There appears to be something strange with your musicTemp variable.

You are hard casting it (which is usually a code smell) and the error is around that variable and the object that it really contains.

The indicator is:

-[MPMediaItemCollection compare:]: unrecognized selector sent to instance 0x14ef90390

Core Data is trying to call compare: on the MPMediaItemCollection object but 0x14ef90390 is not a MPMediaItemCollection.

Start poking at that variable and make sure it really is what you think it is.