how to create a auto-incremented attribute in xcode managed object model

Question

Hey, what i want to do is to make a int that will be the ID of the entity and auto increment itself whenever a new entity is added (just like a SQL identity property). Is this possible? i tried using indexed (checked on attribute) however there is no description to what indexed even does.

EDIT:

I am adding annotations to a map, when you click the pin the annotationview with a chevron shows up. Now when you click this button i would like to load an associated picture. However the only way i know how to link the picture to this button is to give the picture a unique id, and then set the button.tag to this id and then load based on the button.tag.

Answer 1

This kind of concept is contrary to the principles of Core Data - the idea is that you're managing sets of entities with properties, not rows in a database or other things that need to be uniqued. (If you're using the SQLite store, Core Data will create an ID for you behind the scenes, but you can't access it.)

You should probably reconsider (or at least give more details about) the problem you're trying to solve, because as it stands, Core Data will not let you autoincrement a variable.

If you absolutely must, you can manually increment on insert by having some NSNumber ID field on your entity, then every time you insert a new entity, get the existing entities sorted by that ID and limited to one result (using a NSFetchRequest with various options), grab that entity's ID, add one, and set it as the new entity's ID. It's a lot of work, though, and probably error-prone.

Edit: Based on the extra information, I'd say rather than trying to autoincrement an ID yourself, find some other guaranteed-unique property of the annotation and either use that directly or write a hash function that uses it to generate your unique ID. For example, use the latitude & longitude to build a single integer that uniquely represents that point within your system. Other than that, there's no way around having to increment the ID yourself.

I agree that this is a sticky problem - I haven't ever come across something like this in Core Data before, and I can see where autoincrementing might be useful :)