Ios – Coredata delete rule confusion, xcode

core-dataiosiphoneobjective-cxcode

I have a Core Data relationship between two entities, which is like this:

Entity A                        Entity B
aRelationship <-------------->> bRelationship

With the delete rule set to cascade.

Maybe I have this wrong, but I thought that if the delete rule for both of these relationships was set to "Cascade", then when did the following…

[context deleteObject:EntityA];

…it would also delete all the of the Entity B's associated with it. However when I log all of my entity B's it would seem that I am mistaken.

Could someone please shed some light on my confusion?

Thank you very much.

Best Solution

While it's not immediately apparent in the graphical data model editor each recipocal relationship i.e. each

<-->

...is really two separate relationship each with its own delete rule. Delete rules are activate when an object of the entity with the delete rule is deleted.

So, if in the data model editor you have two entities Alpha and Beta with a relationship:

Alpha.betas<-->>Beta.alpha

… then you really have two relationships like so:

Alpha.betas--(delete rule)-->>Beta.alpha
Beta.alpha--(delete rule)-->Alpha.betas

You never want to set up a delete rule like this:

Alpha.betas--(cascade)-->>Beta.alpha
Beta.alpha--(cascade)-->Alpha.betas

… because deleting any one Beta instance will delete the associate Alpha object which will trigger the deletion of all related Beta objects. Depending on the details of your data model, a reciprocal cascade can delete a big chunk of you data by accident.

What you really want is:

Alpha.betas--(cascade)-->>Beta.alpha
Beta.alpha--(nullify)-->Alpha.betas

Now, when you delete the Alpha object, it will delete all associated Beta objects.

When a cascade is blocked, it is usually a problem with a required relationship. Can't tell for certain without details of the data model.

iOS 9 and later:

iOS 9 added a new class called NSBatchDeleteRequest that allows you to easily delete objects matching a predicate without having to load them all in to memory. Here's how you'd use it:

Swift 5

let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

do {
    try myPersistentStoreCoordinator.execute(deleteRequest, with: myContext)
} catch let error as NSError {
    // TODO: handle the error
}

Objective-C

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];

NSError *deleteError = nil;
[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];

More information about batch deletions can be found in the "What's New in Core Data" session from WWDC 2015 (starting at ~14:10).

iOS 8 and earlier:

Fetch 'em all and delete 'em all:

NSFetchRequest *allCars = [[NSFetchRequest alloc] init];
[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];
[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectID

NSError *error = nil;
NSArray *cars = [myContext executeFetchRequest:allCars error:&error];
[allCars release];
//error handling goes here
for (NSManagedObject *car in cars) {
  [myContext deleteObject:car];
}
NSError *saveError = nil;
[myContext save:&saveError];
//more error handling here

Cocoa – How to properly cascade delete managed objects in Core Data

I'm certainly no Core Data expert, but reading the documentation on the various delete rule options, it seems to me that you want the B -> C relationship to be Nullify, rather than No Action. Perhaps the Bs aren't going away because the Cs are still holding references to them?

Best Solution

Related Solutions

Ios – Core Data: Quickest way to delete all instances of an entity

iOS 9 and later:

Swift 5

Objective-C

iOS 8 and earlier:

Cocoa – How to properly cascade delete managed objects in Core Data

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