Passing and Retrieving Core Data - NSArray Element Issue in Swift

Question

I am having an issue with a Swift (2.0) app I am writing in Xcode 7. Basically, I want to save a string to Core Data, and then retrieve it. Sounds really simple but I'm having some issues. The app is a simulated bomb, so I am writing the time it should 'detonate' as a string to Core Data. I then want to retrieve that string and do things with it.

Here is my code to save the string in Core Data:

let dteTime:NSDate = NSDate(timeIntervalSinceNow: 60*60*12) //Generate time 12 hours from now
    let formatter:NSDateFormatter = NSDateFormatter()
    formatter.timeStyle = NSDateFormatterStyle.LongStyle
    formatter.dateStyle = NSDateFormatterStyle.LongStyle
    let strStringValue:String = formatter.stringFromDate(dteTime)
    print(strStringValue) //Prints 'October 20, 2015 at 5:53:18 AM GMT+11' as expected

    let newTime = NSEntityDescription.insertNewObjectForEntityForName("Time", inManagedObjectContext: context)
    newTime.setValue(strStringValue, forKey: "dateTimeExplode")

    do {
        try context.save()
    } catch _ {
        print("Error saving core data")
    }

I have confirmed that calculating the time 12 hours from now is working, and it is formatted into a string correctly.

Below is the code to retrieve the entry from Core Data:

let request = NSFetchRequest(entityName: "Time")
    request.returnsObjectsAsFaults = false

    var results:[TimeCoreData] = [TimeCoreData]()
    results = (try! context.executeFetchRequest(request)) as! [TimeCoreData]

    if results.count > 0 {            // Data retrieved
        var dteTimeExplode:String = String()
        for res in results { //CRASHES

            dteTimeExplode = res.dteTimeDateExplode
        }
}

You'll have to forgive me for any confusion here, but dteTimeExplode is actually a string, not a date. It was originally an NSDate() but I ran into problems, so to simplify it temporarily, I changed it to a string. There may be a few cases like that throughout the code.

As you can see, I am retrieving the Core Data into a custom object, which is defined below. I have copied the entire swift file.

import UIKit
import CoreData
import Foundation

@objc(TimeCoreData)

class TimeCoreData: NSManagedObject {
    @NSManaged var dteTimeDateExplode:String
}

At the moment, the app crashes when it gets to for res in results { (labelled in the second code chunk above) with the error message

fatal error: NSArray element failed to match the Swift Array Element type

I understand that this error message is being received because Swift is trying to make an array of one type out of data of another type and it is unable to do so, but I don't understand why that is the case. I am saving a string into Core Data, so I should be retrieving a string. My custom object (TimeCoreData) has only one variable and that is of type string.

Below is a screenshot of my Core Data model:

What am I doing wrong here?

Why are you re-assigning different values to the `dteTimeExplode` string at each iteration of the loop? — Mundi, Oct 19 '15 at 15:29
@Mundi I am re-assigning values because I want the latest value that is stored, so I loop through until there are no more lines in Core Data. This should only ever be 1 because I truncate the entity before saving, but I am just making sure as I had problems in a previous project. — Matt Kelly, Oct 19 '15 at 22:29

score 0 · Answer 1 · answered Oct 19 '15 at 15:32

0

It looks like you have not told your model that the Time entity should use the TimeCoreData class. I would advise you take the simplest route and refactor by changing the NSManagedObject subclass name to Time.

answered Oct 19 '15 at 15:32

Mundi

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Passing and Retrieving Core Data - NSArray Element Issue in Swift

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