CoreData Swift and transient attribute getters

Question

Any advice on implementing calculated attributes when using Core Data in Swift?

with the generated ManagedObject class, I tried to override the getter but I get the error:

'NSManaged' not allowed on computed properties

which implies you cannot override the getter for a transient (calculated) attribute.

In the code sample below, dateDue is defined as a transient attribute in my model.

Please note that the @NSManaged lines were generated by Xcode - not added by me.

@NSManaged var timeStamp: NSDate
@NSManaged var dateDue: String { 
    get {

        self.willAccessValueForKey("dateDue")
        var ddtmp  = self.primitiveValueForKey("dateDue") as String?
        self.didAccessValueForKey("dateDue")

        if (ddtmp == nil)
        {

            let calendar = NSCalendar.currentCalendar()

            let components = calendar.components((NSCalendarUnit.YearCalendarUnit | NSCalendarUnit.MonthCalendarUnit ) , fromDate: self.timeStamp)
            ddtmp = "\(components.year * 1000 + components.month)"
            self.setPrimitiveValue(ddtmp, forKey: "dateDue")

        }



        return ddtmp!
    }

}

Answer 1

First, in the data model create a transient attribute (section). Because it is transient, it is not physically stored and thus not stored in the managed object context.

The section attribute is shown here:

The entity is shown here:

The class NSManagedObject subclass should have computed 'section' attribute. The NSManagedObject subclass demonstrating how to accomplish this is shown here:

class Number: NSManagedObject {

    @NSManaged var number: NSNumber

    var section: String? {
        return number.intValue >= 60 ? "Pass" : "Fail"
    }
}

Then you must set sectionForKeyPath in the NSFetchedResultsController initializer to be the transient attribute key in the data model and the cache name if desired.

override func viewDidLoad() {
        super.viewDidLoad()

        fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest(), managedObjectContext: managedObjectContext!, sectionNameKeyPath: "section", cacheName: "Root")
        fetchedResultsController?.delegate = self
        fetchedResultsController?.performFetch(nil)

        tableView.reloadData()
}

func fetchRequest() -> NSFetchRequest {

    var fetchRequest = NSFetchRequest(entityName: "Number")
    let sortDescriptor = NSSortDescriptor(key: "number", ascending: false)

    fetchRequest.predicate = nil
    fetchRequest.sortDescriptors = [sortDescriptor]
    fetchRequest.fetchBatchSize = 20

    return fetchRequest
}

The result is a UITableViewController with grades sorted by pass or fail dynamically:

I made a sample project that can be found on GitHub.

Answer 2

"Transient" and "computed" in the sense you mean are different things and are mutually exclusive.

Transient means that the value is stored in memory on the object graph. Computed means that the value is stored nowhere and is calculated in the getter. Both are distinct from the classic non-transient attribute which is stored on the object graph and is saved to disk.

@NSManaged can only be applied to attributes that have a slot in your managed object model.

Answer 3

I have solved this in Swift with an extension, so there is no need to subclass the NSManagedObject and I don't have to generate class files for my models.

So for the above example with class Number, create a file Number+Section.swift you can load the transient property value like this in awakeFromFetch

import Foundation    
extension Number {
    public override func awakeFromFetch() {
        super.awakeFromFetch()
        section = number.intValue >= 60 ? "Pass" : "Fail"
    }
}

I found this way of loading transient fields in Apple's Core Data programming guide.

awakeFromFetch is invoked when an object is reinitialized from a persistent store (during a fetch). You can override awakeFromFetch to, for example, establish transient values and other caches.

Answer 4

Remove the NSManaged attribute.