检查项目是否已保存到 CoreData

Question

我正在将项目保存到收藏夹并在 tableView 中检索它们。但是现在用户可以添加这些项目的副本。我试图通过检查该项目是否已存在于 CoreData 中来避免这种情况，因此防止该项目再次被保存。我怎样才能做到这一点

var item: ItemModel?

    func saveItemsToCoreDataStack(completion: @escaping ()-> Void) {
        managedObjectContext!.perform {
            do {
                try self.managedObjectContext?.save()
                completion()
                print("Item saved to Database")
            } catch {
                print("Sorry Coudn't save itemto Database \(error.localizedDescription)")
            }
        }
    }



var items = [Item]()

var managedObjectContext: NSManagedObjectContext? {
        return (UIApplication.shared.delegate as! AppDelegate ).persistentContainer.viewContext
}

var fetchResultController: NSFetchedResultsController<Item>!

func retrieveitemsFromDatabase() {
        fetchItemFromCoreData { (items) in
            if let item = items {
                self.items = item
                self.tableView.reloadData()
            }
        }
    }

    func fetchItemFromCoreData(completion: ([Item]?)-> Void) {
        var results = [Item]()
        let request: NSFetchRequest<Item> = Item.fetchRequest()
        do {
            results = try managedObjectContext!.fetch(request)
            completion(results)
        } catch {
            print("Sorry couldn't fetch item from Database \(error.localizedDescription)")
        }
    }

Answer 1

这是一个解决方案，您可以通过它防止重复记录进入您的核心数据。

Swift 4.0

        let entityForTableName = NSEntityDescription.entity(forEntityName: "EntityName", in: managedObjectContext)
        let fetchRequest = NSFetchRequest<NSFetchRequestResult>()

        // Predicate with value which are you wan't to be repeated i.e server_id, item_id etc.
        let predicate = NSPredicate.init(format: "yourUniqueKey == %@", <YourUniqueValue>)
        fetchRequest.predicate = predicate
        fetchRequest.entity = entityForTableName

        do {
            let arrData = try managedObjectContext.fetch(fetchRequest)

            if arrData.count > 0 {
                print("Record already exist")
            } else {
                // Write your code here to add record
            }

        } catch {
            print(error.localizedDescription)
        }

客观 C

        // Solution for objective c also
        NSEntityDescription *yourEntityObject = [NSEntityDescription entityForName:@"YourEntityName" inManagedObjectContext:_managedObjectContext];
        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
        [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"uniqueField == %d", <ValueToCheckAlreadyExist>];
        [fetchRequest setEntity:yourEntityObject];

        NSArray *arrData = [_managedObjectContext executeFetchRequest:fetchRequest error:nil];

        if ([arrData count] > 0) {
        // Data already exist
        } else {
        // Create new data
        }