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

iphone xcode core-data nspredicate nsmanagedobject
WoodenKitty picture WoodenKitty · Jan 18, 2011 · Viewed 40.2k times · Source

Suppose I have a list of books stored in Core Data. I want to search for a book by it's primary key ID. I know the sqlite file created by Core Data has an ID column in each table, but this doesn't seem to be exposed to me in anyway.

Does anyone have any recommendations?

Thanks!

Answer

Barry Wark picture Barry Wark · Jan 18, 2011

-[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.

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