Coredata: error in constraints

Question

I have this simple code to save an object in coredata. In my xcatamodel I set a constraint to "firstName": Picture. After deleting all entries in the "Student" Entity, the code works fine. But only for one time. When i´m saving the same set a second time, i´m getting "fatal error".

func saveStudent() {

    let student: Student = NSEntityDescription.insertNewObject(forEntityName: "Student", into: DatabaseController.getContext()) as! Student

                student.firstName = "henry"
                student.lastName = "miller"
                student.age = 22

    DatabaseController.saveContext()
    let fetchRequest: NSFetchRequest<Student> = Student.fetchRequest()

    do {
        let searchResults = try DatabaseController.getContext().fetch(fetchRequest)
        print("number of results: \(searchResults.count)")
        for result in searchResults as [Student] {
            print("\(result.firstName) \(result.lastName) \(result.age)")
        }
    } catch {

        print ("error: \(error)")
    }
}

As far as I´m understanding, the constraint is meant to recognize a potential dublicate. Anyone an idea how to solve this problem? Here´s the error: Picture For any help, thanks in advance.

Answer 1

找到了解决方案：

func saveStudent() {

    let student: Student = NSEntityDescription.insertNewObject(forEntityName: "Student", into: DatabaseController.getContext()) as! Student

                student.firstName = "henry"
                student.lastName = "miller"
                student.age = 22

    DatabaseController.getContext().mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

    DatabaseController.saveContext()
    let fetchRequest: NSFetchRequest<Student> = Student.fetchRequest()

    do {
        let searchResults = try DatabaseController.getContext().fetch(fetchRequest)
        print("number of results: \(searchResults.count)")
        for result in searchResults as [Student] {
            print("\(result.firstName) \(result.lastName) \(result.age)")
        }
    } catch {

        print ("error: \(error)")
    }
}