Swift - Load/save from CoreData generates duplicate entries - ios

Question

I have run into a problem where I can save and load into and from CoreData in Swift for my iOS app, but I run into a problem where I have tried to guard for duplicate entries, but it does not seem to work. can anyone tell me where I went wrong? Thanks!
My ViewController class:
import UIKit
import CoreData
class ViewController: UIViewController, UITableViewDelegate,
UITableViewDataSource {
#IBOutlet weak var headerLabel:UILabel!
#IBOutlet weak var myTableView: UITableView!
var lenders = [LenderData]()
var lendersTemp = [LenderData]()
override func viewDidLoad() {
super.viewDidLoad()
self.myTableView.rowHeight = 90
myTableView.delegate = self
myTableView.dataSource = self
let fetchRequest: NSFetchRequest<LenderData> = LenderData.fetchRequest()
do {
let lenders = try PersistenceService.context.fetch(fetchRequest)
self.lenders = lenders
} catch {
// Who cares....
}
downloadJSON {
for tempLender in self.lendersTemp {
if !self.lenders.contains(where: {$0.id == tempLender.id}) {
self.lenders.append(tempLender)
}
}
self.lendersTemp.removeAll()
PersistenceService.saveContext()
self.myTableView.reloadData()
}
}
func downloadJSON(completed: #escaping () -> ()) {
let url = URL(string: "https://api.kivaws.org/v1/loans/newest.json")
let task = URLSession.shared.dataTask(with: url!) { (data, response, error) in
if error != nil {
print("JSON not downloaded")
} else {
if let content = data {
do {
let myJSONData = try JSONSerialization.jsonObject(with: content, options: JSONSerialization.ReadingOptions.mutableContainers) as AnyObject
var imageID:Int64 = -1
var country:String = "N/A"
var latLongPair:String = "0.000000 0.000000"
var town:String = "N/A"
if let loans = myJSONData["loans"] as? NSArray {
for i in 0...loans.count-1 {
if let lender = loans[i] as? NSDictionary {
if let imageData = lender["image"] as? NSDictionary { imageID = imageData["id"] as! Int64 }
if let countryData = lender["location"] as? NSDictionary {
country = countryData["country"] as! String
town = countryData["town"] as! String
if let geo = countryData["geo"] as? NSDictionary {
latLongPair = geo["pairs"] as! String
}
}
let newLender = LenderData(context: PersistenceService.context)
newLender.id = lender["id"] as! Int64
newLender.name = lender["name"] as? String
newLender.image_id = imageID
newLender.activity = lender["activity"] as? String
newLender.use = lender["use"] as? String
newLender.loan_amount = lender["loan_amount"] as! Int32
newLender.funded_amount = lender["funded_amount"] as! Int32
newLender.country = country
newLender.town = town
newLender.geo_pairs = latLongPair
self.lendersTemp.append(newLender)
}
}
}
DispatchQueue.main.async {
completed()
}
} catch {
print("Error occured \(error)")
}
}
}
}
task.resume()
}
}
EDIT
Added the part of the code where I populate the lendersTemp array

Answer 1

I quote matt on this one from the comments:
So... You are appending to self.lendersTemp on a background thread but reading it on the main thread. Instead, get rid of it and just pass the data right thru the completed function.
Which is exactly what I did. And this worked