NSPersistentDocument FetchRequest warp property crash on macOS Document App SwiftUI project

Question

1. Project create usinging Xcode macOS Document App template, with Use Core Data checkbox checked.
2. Add a Book entity to Document.xcdatamodeld
3. Add FetchRequest warp property to ContentView,

@FetchRequest(entity: Book.entity(), sortDescriptors: []) var books: FetchedResults<Book>

4. Build and Run, Crash!

Crash log from console is

2020-07-03 23:12:23.597880+0800 DocMacDemo[15236:4376209] [error] error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'DocMacDemo.Book' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
CoreData: error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'DocMacDemo.Book' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
2020-07-03 23:12:23.598287+0800 DocMacDemo[15236:4376209] [error] error: +[DocMacDemo.Book entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
CoreData: error: +[DocMacDemo.Book entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
2020-07-03 23:12:23.644491+0800 DocMacDemo[15236:4376209] executeFetchRequest:error: A fetch request must have an entity.
2020-07-03 23:12:23.653769+0800 DocMacDemo[15236:4376209] [error] error: The fetch request's entity 0x600003500420 'Book' appears to be from a different NSManagedObjectModel than this context's
CoreData: error: The fetch request's entity 0x600003500420 'Book' appears to be from a different NSManagedObjectModel than this context's
(lldb)

I have looking for NSPersistentDocument SwiftUI example several days, but could NOT find one. Here are some similar or relation questions. Unfortunately, this problem is not solved.

• Using @fetchRequest(entity: ) for SwiftUI macOS app crashes
• SwiftUI @FetchRequest crashes the app and returns error
• https://developer.apple.com/forums/thread/124656?answerId=417869022#417869022
• https://developer.apple.com/forums/thread/132624

EDIT: Upload this issue project to Github, https://github.com/donly/DocMacDemo.

Answer 1

This is due to emptiness of new document. As in any document-based application you have to prepare some default initial data for new document

Here is possible solution. Tested with Xcode 11.4 / iOS 13.4

in Document.swift

class Document: NSPersistentDocument {

    // .. other code here

    override func makeWindowControllers() {

        // in case of new document create new empty book in context
        // that will be shown in opened document window
        let isNew = self.fileURL == nil
        if isNew {
            _ = Book(context: self.managedObjectContext!)       // << here !!
        }

        let contentView = ContentView().environment(\.managedObjectContext, self.managedObjectContext!)

        // ... other code here

Answer 2

Well, the answer of Asperi is fine but that leaves you with a maybe not needed Book in the object model. Another way is just to save the context at the same place as suggested by Asperi:

    if let context = managedObjectContext {
        try? context.save()
    }

Then there will also be no error message from the FetchRequest.