ios - How to remove applied filter on a NSMutableSet

Question

I have two entities Student and Subjects. Student can have multiple subjects. I am getting NSMutableSet object from Student (NSManagedObject)

 let studentSubjects = student?.mutableSetValue(forKey: "subjects")

Now I want to check if a subject is already belongs to that student. For this I am applying filter on NSMutableSet like this

 let assignedSubjects = assignedSubjects.filtered(using: NSPredicate(format: "subject_title == %@", "Science"))

Now I am checking if count is > 0 then do some action else want to assign the subject to the student

 let subjectEntity = NSEntityDescription.entity(forEntityName: "Subject", in: context)

 var subjectObject = NSManagedObject(entity: subjectEntity!, insertInto: context)

 if assignedSubjects.count > 0 {

      if let filteredItem = assignedSubjects.first as? NSManagedObject {

           subjectObject = filteredItem

      }

 }

 if assignedSubjects.count == 0 {
      if let subjectsManagedObject = studentSubjects.allObjects as? [NSManagedObject] {
           if subjectsManagedObject.count > 0 {
                studentSubjects.add(subjectObject)
           } else {                       
                student.setValue(NSSet(object:subjectObject), forKey: "subjects")
           }
           try context.save()
           }
      } else {
           try subjectObject.managedObjectContext?.save()
      }

The problem is It is always inserting new records instead of saving existing records.

Answer 1

插入新对象的行是这一行：

    var subjectObject = NSManagedObject(entity: subjectEntity!, insertInto: context)

无论当前学生是否已经有相关科目，都会执行。修改您的代码如下：

 var subjectObject : NSManagedObject
 if assignedSubjects.count > 0 {
      if let filteredItem = assignedSubjects.first as? NSManagedObject {
           subjectObject = filteredItem

      }
 }
if assignedSubjects.count == 0 {
    let subjectEntity = NSEntityDescription.entity(forEntityName: "Subject", in: context)
    subjectObject = NSManagedObject(entity: subjectEntity!, insertInto: context)
    // I assume, following your example, you want to add a new subject with title "Science"....
    subjectObject.setValue("Science", forKey: "subject_title")

     if let subjectsManagedObject = studentSubjects.allObjects as? [NSManagedObject] {
           if subjectsManagedObject.count > 0 {
                studentSubjects.add(subjectObject)
           } else {                       
                student.setValue(NSSet(object:subjectObject), forKey: "subjects")
           }
           try context.save()
           }
      } else {
           try subjectObject.managedObjectContext?.save()
      }