Objective-C/iOS: How to remove 'values' from an attribute in CoreData?

Question

pretty new to objective-c and VERY new to CoreData - this question feels ridiculously simple, but I can't figure it out despite 1.5 hours of searching! looking for greater minds.

situation: have an Entity in CoreData ("AssetType"), and that entity has one Attribute ("label"). AssetType has a to-many relationship with another entity ("Items"). Items has a to-one relationship to AssetType there are currently 3 values possible for a "label" - "Electronics", "Furniture", "Jewelry".

goal: very simply, i would like to remove one of the values from all objects. i would prefer this be done all in one go instead of via a for-loop on the "many" objects (eww), but truthfully i'm just lost in CoreData and syntax so whatever you can provide would be awesome.

code structure/background: I can paste more in if needed, but i'm using a generic UITableView + UINavigationItem editButtonItem to execute edits (within a UIPopoverController if that matters) - that's where the delete method is coming from - and i'm capturing it via the tableView:commitEditingStyle:forRowAtIndexPath: delegate/protocol method. this part is not the problem - i know where to put the code, i'm just lost in CoreData. :(

twist: AssetType does not currently have a class - it exists purely as a property in the Item class (and a separate entity in CoreData), which has been fine up until now.. but maybe when i need to manipulate the attributes (e.g. delete them!), this is when i need to introduce its own class? hoping that's not the case.

thanks folks!

Answer 1

Alternatively if you actually want to remove the AssetType altogether, you can set a delete rule in your model for the reverse relationship from AssetType back to Item. In this case it sounds like you would want a Nullify rule,

Then you would simply delete the AssetType object in question which would then automatically nullify all links from Items that had that AssetType on the next save.

// Get descriptions of our entities so we can create some.
NSEntityDescription *assetTypeEntityDescription = [NSEntityDescription entityForName:@"AssetType" inManagedObjectContext:self.managedObjectContext];
NSEntityDescription *itemEntityDescription = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:self.managedObjectContext];

// Create some asset types
AssetType *furnitureAssetType = [[AssetType alloc] initWithEntity:assetTypeEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];
AssetType *electronicsAssetType = [[AssetType alloc] initWithEntity:assetTypeEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];
AssetType *jewelryAssetType = [[AssetType alloc] initWithEntity:assetTypeEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];

furnitureAssetType.label = @"Furniture";
electronicsAssetType.label = @"Electronics";
jewelryAssetType.label = @"Jewelry";

// Create some items
Item *item1 = [[Item alloc] initWithEntity:itemEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];
item1.assetType = furnitureAssetType;

Item *item2 = [[Item alloc] initWithEntity:itemEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];
item2.assetType = electronicsAssetType;

Item *item3 = [[Item alloc] initWithEntity:itemEntityDescription insertIntoManagedObjectContext:self.managedObjectContext];
item3.assetType = jewelryAssetType;

[self.managedObjectContext save:nil];

NSLog(@"item1 asset type is:%@", item1.assetType.label);
// Output: item1 asset type is:Furniture

// Delete the furniture asset type
[self.managedObjectContext deleteObject:furnitureAssetType];

NSLog(@"item1 asset type is:%@", item1.assetType.label);
// Output: item1 asset type is:Furniture

// Save the changes..
[self.managedObjectContext save:nil];    

NSLog(@"item1 asset type is:%@", item1.assetType.label);
// Output: item1 asset type is:(null)
// Because of the delete rule when the furniture object is deleted relationships that pointed to it are nulled out.  
// The furniture asset type no longer exists.  
// There are now only 2 asset types in the persistent store.

Answer 2

Just to be clear: you've two entities, "Asset Type" and "Items", which have a many-to-many relationship. You want to have all items sever their connection with a certain asset type simultaneously and, ideally, you don't want to iterate through items. Have I got it?

If your asset types are genuinely unique (so, there's exactly one that has the label 'Furniture', for example) then you can just unlink that way around — relationships in Core Data are always bidirectional and unlinking from one side automatically unlinks from the other. NSManagedObject subclasses also have accessors that can add or remove things at the end of 'to many' relationships in sets. So e.g.

[furnitureObject removeItems:furnitureObject.items];

That is assuming that your asset type entity has a 'to many' connection named 'items' that links to items.

Otherwise you can run a predicate to get the list of all assets with the string name 'Furniture' and iterate through those, I guess.