core data - the primary key id of a row in the database

-[NSManagedObject objectID] is the unique ID for an object instance in Core Data. It can be serialized via -[NSManagedObjectID URIRepresentation]. You can retrieve the objectID from a persistent store coordinator with -[NSPersistentStoreCoordinator managedObjectIDForURIRepresentation:] and then get the object from a managed object context with -[NSManagedObjectContext objectWithID:].

BUT

You should keep in mind that Core Data is not an ORM. It is an object graph management framework. That is uses SQLite (and unique row IDs) as a backend is purely an implementation detail. The sooner you can get yourself out of the SQL/RDBMS mindset, the faster you will be happy with Core Data. Instead of trying to find an object from a stored ID, consider why you need that object and what object needs it. If an instance of class Foo needs to be able to get to an instance of class Bar, why not just create an association from the Foo to the Bar and set the appropriate Bar instance as the target of the association on the appropriate Foo instance. Let Core Data keep track of object IDs.

As Barry Wark said, remember always that Core Data is not an orm. Pure SQL details are not exposed to the user and every row is just an object. By the way, sometime you should need to access the "primary key", for example when you need to sync the coredata db with external sql databases (in my case I needed it in a callback function to change the state of an object after INSERT it with success in the remote db). In this case, you can use:

objectId=[[[myCoredataObject objectID] URIRepresentation] absoluteString]

that will return a string like: x-coredata://76BA122F-0BF5-4D9D-AE3F-BD321271B004/Object/p521 that is the unique id used by coredata to identify that object.

If you want to get back an object with that unique id:

NSManagedObject *managedObject= [managedObjectContext objectWithID:[persistentStoreCoordinator managedObjectIDForURIRepresentation:[NSURL URLWithString:objectId]]];

NB: Remember that if the receiver has not yet been saved in the CoreData Context, the object ID is a temporary value that will change when the object is saved.

This is the way you can get the object id as String using Swift from a NSManagedObject:

entity.objectID.uriRepresentation().absoluteString