How to assign CoreData many-to-many relationship in Swift?

Question

I have a NSManagedObject class

class Disease: NSManagedObject {
    @NSManaged var diseaseId: String
    @NSManaged var diseaseName: String
    @NSManaged var dogBreed: NSSet
}

How do I add a new relationship to dogBreed? If i change var type to NSMutableSet, the changes are not saved to database. Any ideas?

Answer 1

As of Xcode 7 and Swift 2.0, the release note 17583057 states:

The NSManaged attribute can be used with methods as well as properties, for access to Core Data’s automatically generated Key-Value-Coding-compliant to-many accessors.

@NSManaged var employees: NSSet

@NSManaged func addEmployeesObject(employee: Employee)
@NSManaged func removeEmployeesObject(employee: Employee)
@NSManaged func addEmployees(employees: NSSet)
@NSManaged func removeEmployees(employees: NSSet)

These can be declared in your NSManagedObject subclass. (17583057)

So you just have to declare the following methods and CoreData will take care of the rest:

@NSManaged func addDogBreedsObject(dogBreed: DogBreed)
@NSManaged func removeDogBreedsObject(dogBreed: DogBreed)
@NSManaged func addDogBreeds(dogBreeds: NSSet)
@NSManaged func removeDogBreeds(dogBreeds: NSSet)

Answer 2

Swift cannot generate dynamic runtime accessors due to strict type system. You can create an extension to Disease class and add missed methods manually, here is the code:

extension Disease {
    func addDogBreedObject(value:DogBreed) {
        var items = self.mutableSetValueForKey("dogBreed");
        items.addObject(value)
    }

    func removeDogBreedObject(value:DogBreed) {
        var items = self.mutableSetValueForKey("dogBreed");
        items.removeObject(value)
    }
}

Remarks:

I suggest you to create separate file for extension Disease+CoreData.swift, This should help to keep your code from overrides when you re-generate the CoreData model.

It is sufficient to create relationship in one of managed objects, second one will be updated with back reference. (Same as was with Objective-C)

Important: To make it all work you should verify that class names of entities in you CoreData model includes your module name. E.g. MyProjectName.Disease

Answer inspired by: Setting an NSManagedObject relationship in Swift

Answer 3

If you are looking for a simple approach just use this.

import CoreData
extension NSManagedObject {
    func addObject(value: NSManagedObject, forKey: String) {
        var items = self.mutableSetValueForKey(forKey);
        items.addObject(value)
    }
}

The implementation:

department.addObject(section, forKey: "employees")

Instead of writing the same method on every NSManageObject class that needs it.

Answer 4

Actually you can just write:

@NSManaged var dogBreed: Set<DogBreed>

And use the insert and remove methods of the Set directly.

Answer 5

As an extension to @Naz. And in combination with @Ash's comment. I've found the only working solution for xCode 6.3.1

import CoreData

extension NSManagedObject {
    func addObject(value: NSManagedObject, forKey: String) {
        self.willChangeValueForKey(forKey, withSetMutation: NSKeyValueSetMutationKind.UnionSetMutation, usingObjects: NSSet(object: value) as Set<NSObject>)
        var items = self.mutableSetValueForKey(forKey);
        items.addObject(value)
        self.didChangeValueForKey(forKey, withSetMutation: NSKeyValueSetMutationKind.UnionSetMutation, usingObjects: NSSet(object: value) as Set<NSObject>)
    }
}

You have to call the willChange and didChange handlers in order to correctly add an object to a to-many relationship in Swift.

myManagedObject.addObject(value, forKey: "yourKey")

Answer 6

@Nycen has the best solution for Xcode 7 with the now automatically implemented methods, however note that the NSOrderedSet issue almost unbelievably still exists (see this question), so if using an NSOrderedSet for your relationship instead of an NSSet you will still need to use a workaround.

(I would have commented instead of posting a new answer, however I am lacking in points)

Answer 7

If you need to use an NSOrderedSet instead of NSSet, I've been using the following code which makes it easier to get at the NSMutableOrderedSet methods for adding, inserting, and removing objects.

@NSManaged private var myOrderedSet: NSOrderedSet

var myMutableOrderedSet: NSMutableOrderedSet {
    return self.mutableOrderedSetValueForKey("myOrderedSet")
}