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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: Int64Now I want to fetch all Entities of type "C" wich have a relationship to "A" for a given name of AInstead 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 -1I'm glad about any refinements as well as for a solution.Thanks in advance!