I admit – it might have taken me less time to figure out my runtime exception if I hadn’t just migrated my project to Swift 3 when I encountered the bug.
That’s the problem isn’t it? You go in… you intend to do one thing. Before long, you’ve got 15 files with an
M out to the right. The project builds (finally), but ah – then there’s runtime.
When the crash occurs, you’re never sure if it was an existing problem, or if it was caused by the code conversion. Who knows – maybe it’s both?
Data model Attributes and NSManagedObject property types – Match ‘em!
Whenever you’re creating a subclass of
NSManagedObject for the Entities in your data model, you’re in a mapping process.
Each Attribute on an Entity maps over to a property on the
Not only must the names of those Attributes and properties match, but the Types of each must match as well.
Optionals are Types
String isn’t the same as
Date isn’t the same as
They’re different Types.
So what happens if you specify that a certain Attribute named, say,
createdOn is a
Date with the Optional checkbox checked in the data model designer like this:
And over in the implementation of your
NSManagedObject subclass, you have code that’s written like this:
1@NSManaged var createdOn: Date //instead of Date? (ie, the ? is missing)
What’ll happen? Well, I can tell you what’ll happen. :]
For me, it displayed just the breadcrumb that made me go, “Oh! Let me go check and see if I’ve got a Type mis-match between my data model and my
Interesting… It looks like there was an attempt to go from an
NSDate? (optional) instance to a
The issue isn’t that I’ve got a mismatch between
Date. The runtime can swap those around and substitute them easily.
Rather, it’s that I’m trying to go from optional, where
nil is fine, to non-optional, where
What have I learned?
1 – Map Attributes to
NSManagedObject subclass properties carefully.
2 – Don’t accidentally miss a
? to indicate that a property is optional if I’ve got it marked as optional in the data model
3 – Read the Debug Navigator. It’s not just a list of gibberish – it can actually provide helpful clues so you know where to go look to solve your problem!