Core Data Primary Key

Question

This may seem stupid, but I still couldn't figure out how to mark a attribute as a primary key in the xcdatamodel file. My persistent storage is sqlite file. Can anyone help me?

In that case, how can I "validate" a ID to be unique? Should I write a validation method or something?

Answer 1

Core Data makes its own primary key - you don't have to add one. You can retrieve it with

NSManagedObjectID *moID = [managedObject objectID];

Answer 2

Keep in mind that Core Data is an object-graph persistence framework, not a database. Things like primary keys are abstracted away because they depend on the implementation of the persistent store.

Answer 3

Your options are:

• Use -[NSManagedObject objectID]. Note that this ID is temporary until either the object is saved for the first time or you call -[NSManagedObjectContext obtainPermanentIDsForObjects:error:]
• Use the CFUUID family of functions to generate a UUID for each object in your -awakeFromInsert method
• Create your own primary key-like system that stores an integer in your model and increments it with the creation of each object

There is no good way to validate that a property is unique. The closest you'll get is to ensure it is unique at creation time, and then implement a custom setter method that stops anyone ever changing the ID.