A Debugging Story

Sep 04, 2014

Debugging is arguably the most important skill in a programmers toolkit, yet it's almost never talked about in technical interviews, and it barely gets a mention in most computer science programs.

Here’s a little story about an elusive Swift/CoreData bug I came across and how I got to the bottom of things.

Green, Red, Refactor?

I’ve been working on a small side project in Swift and I just got around to writing some unit tests. Up until this point it has mostly been proof-of-concept, so the code was pretty rough and I was ready to start cleaning it up. One technique I used throughout my NSManagedObject classes looked something like this:

class func userWithUsername(username: String) -> User {
	var user : User?
	let request = NSFetchRequest()
	request.entity = NSEntityDescription.entityForName("User", inManagedObjectContext: CoreDataStack.sharedInstance.managedObjectContext)
	request.predicate = NSPredicate(format: "username = '\(username)'")
	user = CoreDataStack.sharedInstance.managedObjectContext.executeFetchRequest(request, error: nil).last as? User

	if user == nil {
		user =  NSEntityDescription.insertNewObjectForEntityForName("User", inManagedObjectContext: CoreDataStack.sharedInstance.managedObjectContext) as? User
		user?.username = username

	return user!;

This function will give me the User with the given username, or if one doesn’t exist it creates it. I know this code is working fine, but when I write a unit test it faults on the last line, reporting that user was still nil! But how could that be?


The first step I usually take in debugging is isolation: remove as many external pieces as I can so I’m working with the minimum set of variables within the system.

The function above is not particularly well written, as it depends on CoreDataStack.sharedInstance to work properly. It would be far better to implement dependency injection so I could pass in an NSManagedObjectContext. I’d read a bit about race conditions causing problems with SQLite backed stores in unit tests, so I implemented a new NSInMemoryStoreType persistent store for my tests and refactured the function under test a bit. My new function signature looked like this:

class func userWithUsername(username: String, context: NSManagedObjectContext = CoreDataStack.sharedInstance.managedObjectContext) -> User

I can now pass in an NSManagedObjectContext if I want to, or I can rely on the default one provided by my CoreDataStack.sharedInstance. I patted myself on the back and fired off my tests… Only to have them fail in the exact same way.


Another important part of debugging is inspection: getting good information about what’s going in your sysyem. (Hey, another “I” word! I sense a theme!)

Fortunately CoreData gives us some great inspection tools by the way of launch arguments. I set my launch arguments to -com.apple.CoreData.SQLDebug 3 and run the tests again. I watch the first few CoreData arguments scroll through the debugger as the app launches…then nothing as my tests are fired off. Much headscratching later and I come to the conclusion that NSInMemoryStoreType doesn’t support the com.apple.CoreData.SQLDebug argument (although I wasn’t able to find any documentation about this). I ditch the memory store and create a new persistent store just for the tests. Now that I have an actual SQLite database to look at, I find out that my inserts are failing as well as my fetches.

Relevant XKCD

Break It Down

The last technique I used was breaking it down: assume nothing, confirm every step in the process. I’m sorry that didn’t start with “I”. I’m not really big on themes.

I make some headway when I split my insert statement into two:

let entity = NSEntityDescription.entityForName("User", inManagedObjectContext: context)
user =  User(entity: entity, insertIntoManagedObjectContext: context)

My inserts are working now, but my fetch requests are failing… But why? Why did that fix the insert statement? Why did I only have the problem with tests and not in production? And why isn’t my fetch request working?

I break down the fetch request into it’s parts so I can inspect it properly (LLDB still isn’t working right for me as of XCode 6 beta 6). I finally get to this point:

user = context.executeFetchRequest(request, error: error).last as? User


let results = context.executeFetchRequest(request, error: error)
user = results.last as? User

I’m able to confirm that results is getting data! I’m pulling records in my fetch request! But user still gets set to nil!? -cue head pounding on desk-

But then I see it… On closer inspection the results of my fetch request are of type AppNameTests.User. When I run the app the same fetch request returns objects of type AppName.User. The as? User was killing things, since it was not able to cast between AppNameTests.User and AppNameTests.User!

Now that I knew what to look for, I found this handy Stack Overflow question about using core data models in multiple targets in Swift. I remove the app name namespace I added to the data models, and I add @objc(User) to the top of the class and watch all the tests turn green.