NSFetch Request return sum over computed properties

Hi,

I have an iOS App and a Core Data Model with three entities.

Entity A:
     name: String
     value: Int64

Entity B:
     name: String
  
Entity C:
     relationship1: A
     relationship2: B
     value: Int64

Now I want to fetch all Entities of type "C" wich have a relationship to "A" for a given name of A

Instead of a list of those Cs I'd like to return a number: The sum of (C.value * C.relationship1.value) for all fetched Cs. (line 07)

Because I have no idea how to do this using a FetchRequest, I thought I could use a computed property for C ("product") - which works fine - but I can't access it during the FetchRequest. I think it crashes in the line with the "right here" comment.

*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: 'keypath sum not found in entity <NSSQLEntity C id=2>'

import Foundation
import CoreData
@objc(C)
public class C: NSManagedObject {
    @objc dynamic var product : Int64 {
        get {
            if let test = self.relationship1.value {
                return test * self.value
            } else {
                return 0
            }
        }
    }
}

let fetchRequest = NSFetchRequest<NSDictionary>(entityName: "C")
fetchRequest.resultType = .dictionaryResultType


let sumExpressionDesc = NSExpressionDescription()
sumExpressionDesc.name = "returnValue"

let predicate = NSPredicate(format: "C.relationship1.name == %@", name)

let expression = NSExpression(forKeyPath: #keyPath(C.product)) //right here
sumExpressionDesc.expression = NSExpression(forFunction: "sum:", arguments: [expression])
sumExpressionDesc.expressionResultType = .integer32AttributeType

fetchRequest.predicate = predicate
fetchRequest.propertiesToFetch = [sumExpressionDesc]

do {
    let results = try managedObjectContext.fetch(fetchRequest)
    let resultDict = results.first!
    if let value = resultDict["returnValue"], let count = value as? Int {
        return count
    }
} catch {
    print(error)
}

return -1

I'm glad about any refinements as well as for a solution.

Thanks in advance!