Don't want to save data if it already exists in core data

Question

I am able to save and retrieve data from Core Data, but I don't know how to not duplicate or avoid the same saved data in Core Data.i am duplicating the same value but i don't want to save the same value in core data if it already exist. I don't know what code to check for not duplicating it while i save it?

 func fetchDbdata(){
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let managedContext = appDelegate.persistentContainer.viewContext
    let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "Day")
    do {
        let dayList = try managedContext.fetch(fetchRequest)
        for item in dayList{
            let id = item.value(forKeyPath: "id") as? String
            let name = item.value(forKey: "name") as? String
            print(name as Any)
        }
    }
    catch let error as NSError {
        print("\(error), \(error.userInfo)")
    }
}


func saveDaysList(id: String,name: String) {
guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
    return
}
let managedContext = appDelegate.persistentContainer.viewContext
let entity = NSEntityDescription.entity(forEntityName: "Day", in: managedContext)!
let days = NSManagedObject(entity: entity, insertInto: managedContext)
days.setValue(id, forKey: "id")
days.setValue(name, forKey: "name")

do {
    try managedContext.save()
    daysData.append(days)
} catch let error as NSError {
    print("Could not save. \(error), \(error.userInfo)")
}

}

Answer 1

First and foremost, CoreData class of your Day entity should be created. It will be more convenient to get it under control.

class Day: NSManagedObject {
    @NSManaged var id: String
    @NSManaged var name: String
}

Dealing with duplicating problem in Core Data has a common scenario. It is you should first find the same item in the context. For me, most of the time, I will first find the item in caches, and run an NSFetchRequest to fetch it if the preceding operation failed.

I could give some sample methods to you.

public protocol Managed: class, NSFetchRequestResult {
    static var entity: NSEntityDescription { get }
    static var entityName: String { get }
}

extension Managed where Self: NSManagedObject {
    public static func materializedObject(in context: NSManagedObjectContext, matching predicate: NSPredicate) -> Self? {
        for object in context.registeredObjects where !object.isFault {
            guard let result = object as? Self, predicate.evaluate(with: result) else { continue }
            return result
        }
        return nil
    }

    public static func fetch(in context: NSManagedObjectContext, configurationBlock: (NSFetchRequest<Self>) -> () = { _ in }) -> [Self] {
        let request = NSFetchRequest<Self>(entityName: Self.entityName)
        configurationBlock(request)
        return try! context.fetch(request)
    }

    public static func findOrFetch(in context: NSManagedObjectContext, matching predicate: NSPredicate) -> Self? {
        guard let object = materializedObject(in: context, matching: predicate) else {
            return fetch(in: context) { request in
                request.predicate = predicate
                request.returnsObjectsAsFaults = false
                request.fetchLimit = 1
            }.first
        }
        return object
    }
}

extension Day: Managed {

}

Use the findOrFetch method you could try finding the duplicating item. If the item was found, then you should not create a new one. Otherwise you could insert a new item to the db.

Assuming id is the unique property, the finding code will be like below:

let predicate = NSPredicate(format: "id=%@", "your id value")
if let object = Day.findOrFetch(in: context, matching: predicate) {
    // update your old item here.
} else {
    // insert your new item here.
}