Save/Load system for game using CoreData

Question

I've been trying to build a simple load/save system for a game using Core Data.

Saving, loading and creating my UIManagedDocument works fine. Setting and loading values for attributes in my savegame entity works fine as well.

The problem is that these values are lost when my app quits, because I don't know how to access them.

I have about 200 attributes inside my savegame entity, such as (NSNumber *)currentIncome, NSNumber *currentMoney, NSNumber *currentMonth etc. Seems simple, right? If I were to group these into attributes with relationships, I'd probably end up with about 20 attributes. From what I gathered from the Core Data Programming Guide, in order to fill the entity with the values from my saved UIManagedDocument, I need to perform a fetchrequest with a predicate which fills an array of results.

This is where my first question arises: Would I need to perform a fetchrequest for every single attribute? This seems useful if you only have a few attributes or if you have 'to many' relationships. In my case, this would seem incredibly tedious though.

I might be missing something very essential here, but I would need something like load my UIManagedDocument and automatically fill my NSManagedModel with the one that was saved in the document's context and I cannot find it.

That is where my second question comes in: Is CoreData and UIManagedDocument even the right approach for this? 200 variables is too much for NSUserDefaults - I could imagine using NSCoding though. I definately want to incorporate iCloud savegame sharing at a later point, and UIManagedDocument and CoreData just seemed perfect for this.

---

Solved: I just rewrote the entire Core Data fetching code (20 lines down to 10 or so). Performing a fetchrequest for an entity without a predicate apparently returns the entire entity. If my (NSArray *)fetchedResults turns up nil (database is empty), I insert a new instance of my savegame entity in my managedobjectcontext. If it turns up non-nil, I just do a (NSManagedObject *)saveGame = [fetchedResults lastObject] and every value gets loaded fine.

Answer 1

From a database perspective it sounds like what you have here is a database with a single table saveGame with 200 columns currentMoney, currentMonth, etc. You then have a single row in your database representing the current game state.

The NSFetchRequest is the equivalent of the database SELECT statement, and as you only have one row you don't really need any predicates WHERE clauses etc, just get everything from this table, which is what your fetch request that only specifies the entity is doing SELECT * FROM saveGame.

So all in all it doesn't sound like you're getting much value out of the core-data framework here. Another alternative might be to look into the iCloud Key-Value storage API, which sounds closer to what you are currently getting from core-data.