Re: CoreData: Single coordinator, multiple contexts?

FROM : mmalcolm crawford
DATE : Sat Apr 30 01:47:09 2005

On Apr 29, 2005, at 4:03 PM, Paul Mix wrote:

> What I'm curious about is how best to use the new CoreData/
> persistence classes to emulate the typical database-driven 
> approach, where you have a single database file, but with with 
> multiple documents providing interfaces to it.
>

*If I understand correctly what you're after*, in general, although 
Core Data does handle multiple concurrent access (to address one of 
the worries from a week or two ago, it does use optimistic 
locking...) this is not what Core Data is intended for.  If you want 
a database application, use a database.


> As an example, say I'm writing a program to keep track of 
> Personnel. The primary entities would be Person and Group. I'd like 
> to store all these entities in a single SQLite database using 
> CoreData. However, rather than using a single "document" with a 
> master/detail interface, I'd rather have one window showing a list 
> of all Persons or Groups, and a separate editor "document" window 
> to edit each Person or Group. These editor windows should behave 
> just like normal a normal NSPersistentDocument with regards to how 
> it interacts with the store, but I want all of the documents to 
> read and write from the same database file on disk. Changes made in 
> one Person editor would not affect those in any other editor until 
> the document was saved (i.e. the changes committed).
> What's the best approach for handling a paradigm such as this?
>


Xcode already provides a template --  "Core Data Application"-- to 
start down this path.  It provides a single-window application where 
the persistent store coordinator is set up and managed by an 
application delegate.  You could extract the part of the -
managedObjectContext method that creates the coordinator and use it 
to implement a -coordinator method (as illustrated below).  You are 
free then to add as many managed object contexts as you wish...

    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] 
init];
    [moc setPersistentStoreCoordinator:[[NSApp delegate] coordinator];


mmalc



AppDelegate ivar:
    NSPersistentStoreCoordinator *coordinator;


- (NSPersistentStoreCoordinator *) persistentStoreCoordinator {
    if (coordinator != nil) {
        return coordinator;
    }

    NSError *error;
    NSURL *url;

    NSString *path = @"** path to your shared store **";
    // check path exists -- perhaps create it if it doesn't, else 
report an error

    url = [NSURL fileURLWithPath: path];

    coordinator = [[NSPersistentStoreCoordinator alloc]
                initWithManagedObjectModel: [self managedObjectModel]];
    if (![coordinator addPersistentStoreWithType:MY_STORE_TYPE
                                  configuration:nil
                                            URL:url
                                        options:nil
                                          error:&error])
    {
        [[NSApplication sharedApplication] presentError:error];
    }

    return coordinator;
}