How to Use This Fetchrequest() in Swift

How do I use this fetchRequest() in Swift?

You are using two different entities Countries and Atributos in the fetchRequest() declaration. That indeed ambiguous. The entity must be unique.

@nonobjc public class func fetchRequest() -> NSFetchRequest<Countries> {
    return NSFetchRequest<Countries>(entityName: "Countries")
}

and you have to use it

let fetchRequest : NSFetchRequest<Countries> = Countries.fetchRequest()

Note: It's highly recommended to name entities in singular form. Semantically each record represents one Country not one Countries. Only to-many relationships are supposed to be named in plural form.

Swift: CoreData FetchRequest within Class doesn't work

You can't use the @Environment and @FetchRequest outside of a View "properly" @Environment get initialized but usually doesn't update.

To fetch from CoreData you'll have to use the "old way". Attached you'll see the link with more information. It is old but a lot of it still applies.

let moc = …
let employeesFetch = NSFetchRequest(entityName: "Employee")

do {
    let fetchedEmployees = try moc.executeFetchRequest(employeesFetch) as! [EmployeeMO]
} catch {
    fatalError("Failed to fetch employees: \(error)")
}

How to use a @FetchRequest with the new searchable modifier in SwiftUI?

WWDC 2021 Bring Core Data Concurrency to Swift and SwiftUI has a great example of this right around minute 21:33

https://developer.apple.com/wwdc21/10017

struct ContentView: View {
    @FetchRequest(sortDescriptors: [SortDescriptor(\Quake.time, order: .reverse)])
    private var quakes: FetchedResults<Quake>

    @State private var searchText = ""
    var query: Binding<String> {
        Binding {
            searchText
        } set: { newValue in
            searchText = newValue
            quakes.nsPredicate = newValue.isEmpty
                           ? nil
                           : NSPredicate(format: "place CONTAINS %@", newValue)
        }
    }

    var body: some View {
        List(quakes) { quake in
            QuakeRow(quake: quake)
        }
        .searchable(text: query)
    }
}

How to execute Core Data NSFetchRequest in a class method?

let moc = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)

You're making a brand new managed object context here, with no backing persistent store or model. That's not the right way to do it. You need to use the same context you passed in to your SwiftUI environment object, wherever that came from.

You've said that the app delegate has an NSPersistentContainer so I'm guessing you've used the SwiftUI / Core Data app template. In that case you need to replace the line of code above (let moc = ...) with something like this:

let moc = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

If you are using the template, you can look in SceneDelegate.swift and see where the context is injected into the SwiftUI environment using similar code.

How to use @Fetchrequest outside a View

I came across the your question while trying to figure out how achieve a similar result myself. I'm guessing you may have cracked this by now, but for the benefit of those stumbling across the same question I'll have a go at answering, or at least pointing at where the answer might be be found :-)

As has been mentioned the @FetchRequest property wrapper doesn't work outside of View components (which seems a bit of an odd functional omission)

The best alternative I've found is to implement essentially the same thing with a Combine publisher for updates from the NSFetchedResultsController as suggested by Apostolos in his answer here and detailed in a linked Medium post

I've put together a simple proof of concept example that tests and demonstrates the approach with iOS14 and its new life cycle management. This demo app can be found in my GitHub repo over here

Good luck.

How to use a Fetch request from xcdatamodeld?

Bharath's answer is for how to create and execute a fetch request in code. This is commonly how you'll do it. However, it's not actually the answer to your question. If you actually create a fetch request named "allCategories" in the core data model like that, this is how you use it:

let fetchRequest = self.managedObjectModel.fetchRequestTemplate(forName: "allCategories")

let fetchedObjects = self.managedObjectContext.execute(fetchRequest!)

How do I use a NSFetchRequest with an NSExpression and .dictionaryResultType as a SwiftUI @FetchRequest?

I solved this with a Singleton that gets initialized in the app root and injected into each view that needs it as a @StateObject. The Singleton gets access to the NSManagedObjectContext via a one-time setup method. It uses the NotificationCenter via NSManagedObjectContext.didSaveObjectsNotification to listen to model changes and recalculate. While a Singleton isn't really popular to use in SwiftUI (often for good reason!) this one does not affect the underlying data, it just enriches and presents it.

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