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.

Do you want to remove the applied filter, so can't you just remove the line where you filter using predicate? I don't get the point. — kamil3, Jul 20 '17 at 15:35
I cant remove the line, I have to check if the record already exists or not. — Rajat Jain, Jul 20 '17 at 15:46
When i m again trying to fetch assigned subjects, I see that the filter is applied — Rajat Jain, Jul 20 '17 at 15:48
`filter(using: predicate)` mutates the set, but `filtered(using: predicate)` returns a completely new set containing only the items that match the predicate. So use `let filteredSet = assignedSubjects.filtered(using: predicate)`, then test the count of `filteredSet`. — pbasdf, Jul 20 '17 at 21:21
@pbasdf your suggestion worked for me. Now I am trying to update the existing subject record. if let filteredItem = filteredItems.first as? NSManagedObject { subjectObject = filteredItem } This always insert a new object in the table without relationship — Rajat Jain, Jul 21 '17 at 07:35
Please can you edit your question to show your revised code? — pbasdf, Jul 21 '17 at 07:56

score 0 · Accepted Answer · answered Jul 21 '17 at 11:07

The line which inserts a new object is this one:

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

which is executed regardless of whether the current student already has the relevant subject. Amend your code as follows:

 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()
      }

I don't want to add a new subject. If Science already assigned to student then I want to update some columns of that record. — Rajat Jain, Jul 21 '17 at 11:12
In that case, just delete the entire second if statement (`if assignedSubjects.count == 0 { ... }`). Update the attributes after `subjectObject = filteredItem`. — pbasdf, Jul 21 '17 at 11:19
After `subjectObject = filteredItem` when I am trying to save the context with `try context.save()` its inserting new records instead of updating existing record. — Rajat Jain, Jul 21 '17 at 11:48

How to remove applied filter on a NSMutableSet

1 Answers1