swift 2 core data predicate inconsistent - ios

Question

I am having an issue with core data's predicate method, in that it seems to be somewhat inconsistent in the results it is returning. I have a "Users" entity that is populated from a web API, rather than deleting all the entries then downloading and adding the users, I prefer to set a flag for all users "isModified" to false, then download the users over the API and update or add users depending on whether they are in the store or not, setting the "isModified" flag when they are added/updated. Finally the code deletes any users that did not have their "isModified" flag set. I do it this way because in the event that the web API is not available I don't lose all my users and the app can continue to work.
The problem I am having is that the method that deletes users that haven't had their "isModified" flag set is deleting users that HAVE been updated!
Here's the method:
func deleteUsersNotUpdated() -> Bool {
// default result
var result = false
// setup the fetch request
let fetchRequest = NSFetchRequest(entityName: "Users")
// set a predicate that filters only records with updated set to false
fetchRequest.predicate = NSPredicate(format: "isModified == %#", false)
do {
let fetchResults = try self.managedObjectContext.executeFetchRequest(fetchRequest) as! [NSManagedObject]
for user in fetchResults {
print("Deleting \(user)")
self.managedObjectContext.deleteObject(user)
try self.managedObjectContext.save()
}
result = true
} catch let error as NSError {
print("\(error)")
}
return result
}
The method mostly works, but intermittently will delete a user for no good reason, i.e. even though it has the "isModified" flag set, e.g. here is the output of the line: print("Deleting (user)")
Deleting <NSManagedObject: 0x7b6d6ec0> (entity: Users; id: 0x7b64abd0 <x-coredata://1A7606EB-3539-4E85-BE1C-15C722AD7125/Users/p14> ; data: {
created = "2016-01-17 16:54:21 +0000";
familyName = Doe;
givenName = John;
isModified = 1;
pin = 3932;
type = STAFF;
})
As you can see, the "isModified" flag is very definitely set, yet the predicate is supposed to select only records that have the flag reset (false).
The method above is part of a class I have created, it's basically a CRUD class for the Users entity.
Scratching my head here!
I can supply the rest of the code if required.

Answer 1

I think your code looks perfectly fine (although you should save outside the loop). Swift booleans are automatically converted to NSNumber and back in most situations. There are many valid ways to write this predicate.
The only possible explanation that comes to mind is that another class is using the same managed object context to modify the fetched objects. If you have a single context app this a conceivable scenario.
The standard way to avoid this is to make the changes on a background context.

Answer 2

Use an boolean literal rather than a object placeholder %# for a boolean value
NSPredicate(format: "isModified == FALSE")
and don't call context.save() in each iteration of the repeat loop, call it once after the repeat loop.