How can I create instances of managed object subclasses in a NSManagedObject Swift extension?

(Updated for Swift 3/4 now. Solutions for earlier Swift versionscan be found in the edit history.)

You can use unsafeDowncast to cast the return valueof NSEntityDescription.insertNewObject() to Self(which is the type on which the method is actually called):

extension NSManagedObject {    class func create(in context: NSManagedObjectContext) -> Self {        let classname = entityName()        let object = NSEntityDescription.insertNewObject(forEntityName: classname, into: context)        return unsafeDowncast(object, to: self)    }    // Returns the unqualified class name, i.e. the last component.    // Can be overridden in a subclass.    class func entityName() -> String {        return String(describing: self)    }}

Then

let obj = YourEntity.createInContext(context)

works and the compiler infers the type of obj correctly as YourEntity.

Here is a different approach to solve the problem, by implementingan initializer method (tested with Xcode 7.1):

extension NSManagedObject {    // Returns the unqualified class name, i.e. the last component.    // Can be overridden in a subclass.    class func entityName() -> String {        return String(self)    }    convenience init(context: NSManagedObjectContext) {        let eName = self.dynamicType.entityName()        let entity = NSEntityDescription.entityForName(eName, inManagedObjectContext: context)!        self.init(entity: entity, insertIntoManagedObjectContext: context)    }}

Init methods have an implicit return type of Self and no castingtricks are necessary.

let obj = YourEntity(context: context)

creates an object of the YourEntity type.

Swift 3/4 update:

extension NSManagedObject {    // Returns the unqualified class name, i.e. the last component.    // Can be overridden in a subclass.    class func entityName() -> String {        return String(describing: self)    }    convenience init(context: NSManagedObjectContext) {        let eName = type(of: self).entityName()        let entity = NSEntityDescription.entity(forEntityName: eName, in: context)!        self.init(entity: entity, insertInto: context)    }}

In Swift 2 there is a very smart solution using a protocol and a protocol extension

protocol Fetchable{  typealias FetchableType: NSManagedObject  static var entityName : String { get }  static func createInContext(context: NSManagedObjectContext) ->  FetchableType}extension Fetchable where Self : NSManagedObject, FetchableType == Self{  static func createInContext(context: NSManagedObjectContext) -> FetchableType  {    return NSEntityDescription.insertNewObjectForEntityForName(entityName, inManagedObjectContext: context) as! FetchableType  }}

In each NSManagedObject subclass add the protocol Fetchable and implement the property entityName.

Now the function MyEntity.createInContext(…) will return the proper type without further type casting.