I am working with several NSManagedObject
types with several relationships. How can I tell Core Data to automatically populate object IDs for me? I'm looking for something like an index key in SQL, so that no two instances of a given object are allowed to have the same ID.
Edit:
I'd like for all of my "Account" objects to have unique IDs on them. I was just adding one to the `countForFetchRequest, but I realized that when deleting the second to last object and then adding one, the last two objects now have the same IDs.
How can I ensure that a given value has a unique value for all instances of my "Account" NSManagedObject?
EDIT2:
I need to have a separate ID for sorting purposes.
The way I resolved this is with Core Data aggregates. I actually end up assigning the ID myself.
Essentially, I query Core Data for all of the entity IDs of my entity and then iterate through them. If I find an ID which is higher than the current temporary one, I make the temporary ID higher one higher than the aggregated one. When I'm done, I automatically have an ID which is higher than the highest one in the list. The only flaw I see with this is if there is a missing ID. (I believe that there is a simple fix for this as well.)
//
// Create a new entity description
//
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:self.managedObjectContext];
//
// Set the fetch request
//
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:entity];
//
// We need to figure out how many
// existing groups there are so that
// we can set the proper ID.
//
// To do so, we use an aggregated request.
//
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:@"entityID"]];
NSError *error = nil;
NSArray *existingIDs = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (error != nil) {
//
// TODO: Handle error.
//
NSLog(@"Error: %@", [error localizedDescription]);
}
NSInteger newID = 0;
for (NSDictionary *dict in existingIDs) {
NSInteger IDToCompare = [[dict valueForKey:@"entityID"] integerValue];
if (IDToCompare >= newID) {
newID = IDToCompare + 1;
}
}
//
// Create the actual entity
//
MyEntity *newEntity = [[MyEntity alloc] initWithEntity:entity insertIntoManagedObjectContext:self.managedObjectContext];
//
// Set the ID of the new entity
//
[newEntity setEntityID:[NSNumber numberWithInteger:newID]];
//
// ... More Code ...
//