SwiftUI FetchRequest Error: 'A fetch request must have an entity'

Question

I am building a small app using SwiftUI and CoreData with the SwiftUI App life cycle(no scene or app delegate). I'm getting the following error when I Run+Build my app:

'executeFetchRequest:error: A fetch request must have an entity.'

I've checked/verified/re-checked the following:

My [app name].xcdatamodeld file name is the same as what I pass into the NSPersistentContainer NSPersistentCloudKitContainer(name: [app name])
The entity's name Car is exactly what I pass into the FetchRequest

@FetchRequest(entity: Car.entity(), sortDescriptors: []) var car: FetchedResults<Car>

I am selecting Manual/None for my entities Codegen and the generated class is

public class Car: NSManagedObject {}

with an extension on Car of Identifiable.

Here is my whole view struct that should be(to my understanding) passing the environment around to all of its "child" views.

struct AppView: View {
    
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: Car.entity(), sortDescriptors: []) var car: FetchedResults<Car>
    
    var body: some View {
        List {
            ForEach(car, id:\.self) { item in
                RootView(carData: item)
                    .environment(\.managedObjectContext, self.moc)
            }
        }
    }
}

and my @main struct

@main
struct AutoMateApp: App {
    
    @StateObject var coreData = PersistentCloudKitContainer()
    
    let persistence = PersistentCloudKitContainer()
    
    var body: some Scene {
        WindowGroup {
            AppView()
                .environment(\.managedObjectContext, coreData.persistentContainer.viewContext)
        }
    }
}

When I step through with the debugger the crash seems to appear once the WindowGroup is returned. I'm not sure if that's helpful information at all.

I appreciate all your help, thank you.

I am having the exact same issue. But I am using the Xcode codegen for my Core Data class. Have you found a solution? — jns_ai_unr
– jns_ai_unr, Commented Aug 31, 2020 at 7:31
I have the same error after I switched from codegen to manual/none mode. The app in the sim works fine but previews are crashing with A fetch request must have an entity. — Nik
– Nik, Commented Jun 9, 2022 at 13:26

jrturton · Accepted Answer · 2022-11-30 13:17:41Z

3

I encountered this issue today and I think the problem comes from when things are declared and ordered. Removing the entity from the fetch request (which presumably allows it to do things by type inference, at a later point in the lifecycle) removed the problem:

@FetchRequest(sortDescriptors: [...], predicate: ...) 
private var things: FetchedResults<Thing>

It seems possible that Car.entity() is evaluated before the environment context is created and injected when you are setting up the preview.

answered Nov 30, 2022 at 13:17

jrturton

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jns_ai_unr · Accepted Answer · 2020-08-31 09:49:50Z

2

Okay so removing the lazy from the line lazy var persistentContainer: NSPersistentCloudKitContainer = {...}() in the AppDelegate helped in my case. Because it seemed that Apples boilerplate code handed of the NSManagedObjectModel in the line let contentView = ContentView().environment(\.managedObjectContext, persistentContainer.viewContext) and the FetchRequest accessed that object before it was properly instantiated by the closure and therefore would not be able to find the NSManagedObject subclass. Removing the lazy from the variable executes the closure right away.

Hope this still helps you and solves the problem in your case.

edited Aug 31, 2020 at 9:49

answered Aug 31, 2020 at 9:33

jns_ai_unr

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3 Comments

glob Over a year ago

Thanks for the reply. Apple finally showed us their implementation of how CoreData works with a SwiftUI lifecycle app in their eyes. This appeared in Xcode 12 beta 6. I've since used that and a slightly different approach to the way my app flows which solved my problem.

Nik Over a year ago

removing lazy didn't help me

Misael Landeros Over a year ago

thanks this help me, strange because I have older projects where this work just fine

malhal · Accepted Answer · 2020-09-01 05:25:16Z

Try the code from the SwiftUI app project template when Core Data support is checked:

MyApp.swift

import SwiftUI

@main
struct MyApp: App {
    let persistenceController = PersistenceController.shared

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, persistenceController.container.viewContext)
        }
    }
}

Persistance.swift

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        for _ in 0..<10 {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()
        }
        do {
            try viewContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
        return result
    }()

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "MyApp")
        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                /*
                Typical reasons for an error here include:
                * The parent directory does not exist, cannot be created, or disallows writing.
                * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                * The device is out of space.
                * The store could not be migrated to the current model version.
                Check the error message to determine what the actual problem was.
                */
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
    }
}

Thank you for your response. Since this question was asked before Xcode 12 beta 6 Apple didn't have this boilerplate template yet. Since beta 6 I have used that with a different approach to my app anyways. I appreciate the help. EDIT: Marking this as the right answer since it does suggest what I began doing once beta 6 came out.

tsuyoshi hyuga · Accepted Answer · 2020-10-16 21:37:17Z

0

Replace Car.entity() to static NSEntityDescription.

example

struct AppView: View {
    
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: AppView_Previews.entity, sortDescriptors: []) var car: FetchedResults<Car>
    
    var body: some View {
        List {
            ForEach(car, id:\.self) { item in
                RootView(carData: item)
                    .environment(\.managedObjectContext, self.moc)
            }
        }
    }
}

struct AppView_Previews: PreviewProvider {
    static var entity: NSEntityDescription {
        return NSEntityDescription.entity(forEntityName: "Car", in: yourViewContext)!
    }
}

answered Oct 16, 2020 at 21:37

tsuyoshi hyuga

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1 Comment

chornbe Over a year ago

How is that supposed to work in a final build?

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