Swift 3 Core Data - Nsexpression Forfunction: "Sum:" Throws Error ("Could Not Cast Dictionary to Day")

Swift 3 Core Data - NSExpression forFunction: sum: throws error ( could not cast dictionary to Day )

This solves my problem:

let sumRequest: NSFetchRequest<NSFetchRequestResult> = Day.fetchRequest()

Instead of putting Dayas NSFetchRequestResult, I put NSFetchRequestResult itself there. Now it's working.

I believe as I set it to a different resultType it's not [Day]anymore that gets returned, so that's why I needed to change that to be more generic or something.

xcode8 beta4 not generating the managedObjectContext

The error message reads:

Value of type 'ViewController' has no member 'managedObjectContext'

What this means is that somewhere (your screenshot does not show where) you are attempting to assign a value to a property named managedObjectContext, on an instance of your ViewController class. Except that this class doesn't have a property named managedObjectContext, so Swift complains and doesn't compile.

This is not actually a Core Data question-- it's basic Swift. It would happen for any attempt to assign a value to a nonexistent property.

You probably (again, your screenshot does not provide enough detail to be sure) need to create this property on your view controller class, with type NSManagedObjectContext.

Parsing NSDictionary result of grouped by fetch request in Core Data

First of all the result of a Core Data fetch is definitely not a JSON object

Second of all be more specific!

Since the return type of the fetch request is clearly dictionary use the generic type to pass NSDictionary

let request = NSFetchRequest<NSDictionary>(entityName: "GuestsTable")

Then cast the type accordingly to the actual type [[String:Any]].

var matchedGuests = [[String:Any]]()
...
if let result = try? context.fetch(request) as! [[String:Any]] {
   matchedGuests = result
} 

Even if an try? error occurs the forced unwrapped return type will never cause a crash assuming the fetch request is valid.

---

If you want only the guest names map the array

let popularGuestNames = matchedGuests.flatMap { $0["guestName"] as? String }

How to sum filtered Core Data in SwiftUI?

I have found similar problem in this question:
Sum of (double) fetch request field
I have modified the code a little and came up with this solution:

import SwiftUI
import CoreData

struct DashboardView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    // FetchRequest with predicate set to "after now" 
    @FetchRequest(entity: NPTransaction.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \NPTransaction.value, ascending: false)], predicate: NSPredicate(format: "date > %@", Date() as NSDate)) var fetchRequest: FetchedResults<NPTransaction>

    // sum results using reduce
    var sum: Int64 {
        fetchRequest.reduce(0) { $0 + $1.value }
    }

    var body: some View {
        NavigationView {
            VStack(alignment: .leading) {

                HStack {
                    VStack(alignment: .leading) {
                        Text("sum")
                        Text("\(sum)")
                            .font(.largeTitle)

                    }
                    Spacer()
                }
                .padding()

                Spacer()
            }.navigationBarTitle("Title")
        }
    }

}

struct DashboardView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return DashboardView().environment(\.managedObjectContext, context)
    }
}

Core Data - How to fetch an entity with max value property

You set the fetchLimit to 1 and sort by personId in descending order. E.g.:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Person"];

fetchRequest.fetchLimit = 1;
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"personId" ascending:NO]];

NSError *error = nil;

id person = [managedObjectContext executeFetchRequest:fetchRequest error:&error].firstObject;

Using sum aggregate with CoreData

First, this should be a class method. You are fetching arbitrary Operation instances, so there is no good argument why this should be called in the context of one particular instance.

Second, I think this can be simplified greatly as follows: fetch all operations with the given constraints, then:

let q1Sum = (operations as NSArray).valueForKeyPath("@sum.amount").doubleValue

or, more "swiftly":

let q1Sum = operations.map { $0.amount.doubleValue }.reduce(0, combine: +)

There is no good discernible reason for going into NSExpression complexities and for using NSDictionaryResultType, unless you are getting performance issues, which would be highly unlikely.

How to sum a double attribute from CoreData in swift

In your current code, you're attempting to cast logsArray as an array of doubles when it's in fact an array of NSManagedObjects. That's why you're getting an error when you attempt to reduce the array.

To get the sum of the double values associated with your "totalWorkTimeInHours" key in Core Data, you have to access the "totalWorkTimeInHours" key from each NSManagedObject returned from your fetch request then add them together, ex:

override func viewDidLoad() {
    super.viewDidLoad()

    //CoreData
    let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate
    let managedContext : NSManagedObjectContext = appDelegate.managedObjectContext!
    var fetchRequest = NSFetchRequest(entityName: "Log")
    fetchRequest.returnsObjectsAsFaults = false;
    var results: NSArray = managedContext.executeFetchRequest(fetchRequest, error: nil)!

    var totalHoursWorkedSum: Double = 0
    for res in results {
        var totalWorkTimeInHours = res.valueForKey("totalWorkTimeInHours") as Double
        totalHoursWorkedSum += totalWorkTimeInHours
    }

    print("Sum = \(totalHoursWorkedSum)")
}

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