Question

I’m wanting to build an app with 2 core data handlers for one iphone app.

The app will be a sports game with pre-filled information. Lets call it prefilledDB as a reference. This prefilledDB will be read-only, I do not want the user to add/edit/delete or change anything in this.

The second db (“gameDB”) would have the same core data relationships/models/entities and structure.

When a user selects “New game”, it will blank/empty the gameDB and fill it up with the prefilledDB contents, and “Continue game” would just use prefilledDB, assuming it is not empty.

However, I’m not sure if this is the right way to do it. My question therefore is what is the best way to handle this kind of processing. Would an in-built migration system be better than dropping/recreating databases — or perhaps just using SQLite as the prefilledDB and then filling the gameDB with its contents?

Any help on this would be great.

Answer 1

The prefilled persistent store will have to be readonly if it ships in the app bundle as everything in the app bundle is read only. To make use of the data in a readwrite persistent store, you will need to copy the data to persistent store outside the app bundle e.g. in the documents directory for the app.

You have two ways of doing this:

1) Simplest: Create a new persistent store for each game. In this case you would just copy the prefilled persistent store file from the app directory to the documents directory while in the process renaming it to the current game name. Then you would open that file as the gameDB and it would be automatically populated with the preexisting data. The downside of this system is that you have a large number of files, one for each game, and the data could not be easily shared between them.

2) Best: You use two persistent stores simultaneously in the same context. This is more complicated to set up but gives you greater flexibility. The trick is to create a single abstract subentity e.g. Opponent and then two concrete subentities that are identical save for their names e.g. PreOpponent and GameOpponent. Using the configurations option in the data model, assign PreOpponent to the prefilled persistent store and GameOpponent to the gameDB persistent store. Write some code in the Opponentclass to clone itself to a new GameOpponent instance. When a new game starts, clone all the instances from prefilled to ‘gameDB’. All the newly cloned GameOpponent instances will automatically write to the gameDB persistent store.

The advantage of the latter system is that you have all your active data in one readwrite persistent store which you can then manipulate as needed. E.g. you could check previous game or even clone a previous game to create a novel starting point for a new game.

Marcus Zarrus had some code on his site showing how to set up automatic cloning for managed objects.