CoreData bug in Xcode 12.1

Question

I am working through Paul Hudson's 100 Days of SwiftUI and on Project 11 have hit a frustrating issue with CoreData. This is a direct lift of Paul's code that compiles and runs fine in his video. The Bookworm.xcdatamodeld has a single entity named Student that has two attributes: a UUID named id and a String named name.

It compiles fine, but running it results in a crash on the ForEach, with 'students' underlined in red. The error message that pops up in the console says:

2020-10-31 12:13:47.934507-0400 Bookworm[614:7766183] [error] error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'Bookworm.Student' so +entity is confused. Have you loaded your NSManagedObjectModel yet ? CoreData: error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'Bookworm.Student' so +entity is confused. Have you loaded your NSManagedObjectModel yet ? 2020-10-31 12:13:47.934651-0400 Bookworm[614:7766183] [error] error: +[Bookworm.Student entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass CoreData: error: +[Bookworm.Student entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass 2020-10-31 12:13:47.953419-0400 Bookworm[614:7766183] [SwiftUI] Context in environment is not connected to a persistent store coordinator: <NSManagedObjectContext: 0x6000008d0820>

I have searched a ton, and tried every recommended solution that I have found including: simply closing and reopening Xcode (Step 1), cleaning the project and then repeating Step 1, and deleting all the derived data and repeating Step 1. I have verified that Current Product Module is selected in the inspector for the Module, and that Codegen has Class Definition selected.

import SwiftUI
import CoreData

struct ContentView: View {

    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: Student.entity(), sortDescriptors: []) var students: FetchedResults<Student>

    var body: some View {
        VStack {
            List {
                ForEach(students, id: \.id) { student in
                    Text(student.name ?? "Unknown")
                }
            }
        }
    }
 }

Answer 1

If you are using SwiftUI lifecycle, you should initialize NSPersistentContainer in a parent View (or App) and import managedObjectContext to the environment.

In your case, it could be something like this:

import SwiftUI
import CoreData

@main
struct coreDataParadigmApp: App {
    let persistenceController = PersistenceController.shared
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, persistenceController.container.viewContext)
        }
    }
}


struct ContentView: View {
    @FetchRequest(entity: Student.entity(), sortDescriptors: []) var students: FetchedResults<Student>
    var body: some View {
        VStack {
            List {
                ForEach(students, id: \.id) { student in
                    Text(student.name ?? "Unknown")
                }
            }
        }
    }
 }

// DONT FORGET TO CHANGE THE NAME OF YOUR FILE
struct PersistenceController {
    static let shared = PersistenceController()

    let container: NSPersistentContainer

    init() {
        container = NSPersistentContainer(name: "coreDataNameOfFile")

        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
    }
}