Core Data: setPrimitiveValue:forKey: behaves really weirdly - ios

Question

This is a mystery:
I'm invoking setPrimitiveValue:forKey: on an NSManagedObject. The key is a legit, persistent, modeled attribute of the object. However, setPrimitiveValue:forKey: fails, often setting the value for a different, arbitrary attribute. The docs say this behavior is expected when invoking setPrimitiveValue:forKey: for an unmodeled key. So it seems Core Data thinks the key is unmodeled.
The strange part:
When the key is hardcoded as a string literal, the primitive value is indeed set successfully. It only fails when the key is a variable. The variable I'm using happens to be passed from the keyPath argument of observeValueForKeyPath:ofObject:change:context:
The keyPath variable is the same as the string literal. isEqual: returns true and the hash values are equal. The keyPath variable is of type __NSCFString. Does anyone know why setPrimitiveValue:forKey: would behave any differently? (This behavior is on OS X 10.9.1)
An update with better information:
The misbehaving key traced back to a string loaded from a file on disk. The example below is an isolated case. If the attribute string "mainAttr" is written to disk and read back in, then setPrimitiveValue:forKey: sets the value for the wrong attribute, not "mainAttr".
Core data object:
#interface Boo : NSManagedObject
#property (nonatomic, retain) NSNumber * mainAttr;
#property (nonatomic, retain) NSNumber * attr1;
#property (nonatomic, retain) NSNumber * attr2;
#property (nonatomic, retain) NSNumber * attr3;
#end
-
#import "Boo.h"
int main(int argc, const char * argv[]) {
#autoreleasepool {
NSManagedObjectContext *context = managedObjectContext();
NSString *key = #"mainAttr";
// write to disk, read back in
NSString *path = [#"~/Desktop/test.txt" stringByExpandingTildeInPath];
[key writeToFile:path atomically:YES encoding:NSUTF8StringEncoding error:NULL];
key = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:NULL];
Boo *boo = [NSEntityDescription insertNewObjectForEntityForName:#"Boo" inManagedObjectContext:context];
[boo setPrimitiveValue:#(5) forKey:key];
NSLog(#"Boo: %#", boo);
}
return 0;
}

Answer 1

You need the below 3 statements to set the value. Try it.
[self willChangeValueForKey:key];
[boo setPrimitiveValue:#(5) forKey:key];
[self didChangeValueForKey:key];