I have a Core Data stack based on the NSInMemoryStoreType store. And I've noticed that deleting objects doesn't really remove them or make them nil, bur rather simply turns them into faults.
For example, (MyManagedObjectEntityClass as well as the <> identifier are placeholders):
MyManagedObjectEntityClass *o = [NSEntityDescription insertNewObjectForEntityForName:#"<MyManagedObjectEntityClass Entity Name>" inManagedObjectContext:self.localContext];
NSLog(#"\n%#", o);
[self.localContext deleteObject:o];
NSLog(#"\n%#", o);
Will log that the object is still there only that it's data is a fault.
And adding [self.localContext save:nil]; after the delete doesn't change this either.
I was hoping I could at some point test the o variable for nil, in which case I'd reload the object - but it seems I can't.
Just in case, yes, I know I could instead test o for -isFault. But thing is, extrapolate this test to an NSSet and I can't just rely on [[set anyObject] isFault] to conclude that all objects in that set have been removed (Ideally the set's count would be 0, but all objects are still there as faults).
So I'm wondering if it's possible at all or what alternate approach could I take to be able to test that objects have been deleted in a way transparent to the fact that they are managed objects.
This is not actually a Core Data issue. C (and by extension Objective-C) doesn't work like that.
The deleteObject: method takes one argument, a pointer to an object. It can change the object (like setting its isDeleted flag), or it can do other things related to the object (like deleting it from its managed object context). It cannot change the value of the pointer itself. So whatever it does or should do, C says that once it's done, the pointer that you pass in still points to the same location in memory. As a result it's actually impossible for that method to force that pointer to be nil in this language. If you want it to be nil, you have to change that yourself. (As an aside, it would have been possible to implement the method to take a pointer to pointer argument, which could modify your pointer. This would have no effect on other references such as those in arrays, though, so it would be kind of pointless).
This is why the isDeleted method is public, so that if you have a pointer to this object in some other location, you can check whether it has been deleted before attempting to use it.
If that's not convenient enough (and it often isn't), Core Data also provides NSManagedObjectContextObjectsDidChangeNotification and NSManagedObjectContextDidSaveNotification. You can use these anywhere in your app to get notified of changes to the context and respond in whatever way is appropriate (updating an array, for example). These notifications both try to help you out by providing lists of inserted, updated, and deleted objects. Use those when possible to check whether you actually need to update your references.
Related
Based on some limited testing, I see that if I
Execute a Fetch request with result type = NSDictionaryResultType
Do some manipulations on the returned values
Store back the MOC on which Fetch request was executed
the changes in step 2 are not written back to the persistent store because I am changing a dictionary and not a "managed object". Is that a correct understanding?
Most likely you are abusing the dictionary result type. Unlike in conventional database programming, you are not wasting valuable memory resources when fetching the entire objects rather than just one selected attributes, due to an under-the-hood mechanism called "faulting".
Try fetching with managed object result type (default) and you can very easily manipulate your objects and save them back to Core Data. You would not need to do an additional fetch just to get the object you want to change.
Consider dictionaries only in special situations with huge data volumes, difficult relational grouping logic, etc., which make it absolutely necessary.
(That being said, it is unlikely that it is ever absolutely necessary. I have yet to encounter a case where the necessity of dictionaries for fetches was not an indirect result of flawed data model design.)
Yes, kind of, you can't store a dictionary back into the context directly so you can't save any updates that way.
If you get a dictionary object then you need to include in it the associated managed object id (if it isn't aggregated) or do another fetch to get the object(s) to update.
I'm using CoreData to persist a list of messages in a conversation.
Conversation is a managedObject that has an array of Messages.
In one scenario, I'm trying to delete all the messages in a conversation.
for (UQMessage * message in self.tempConversation.chatMessages){
[self.tempConversation.managedObjectContext deleteObject:message];
error = nil;
[self.tempConversation.managedObjectContext.persistentStoreCoordinator lock];
if (![self.tempConversation.managedObjectContext save:&error]) {
NSLog(#"Can't Delete! %# %#", error, [error localizedDescription]);
return;
}
[self.tempConversation.managedObjectContext.persistentStoreCoordinator unlock];
}
When I check for
self.tempConversation.chatMessages.count
Nothing changes.
Everything works perfectly well when I try to add messages, and when I delete the conversation itself. But I can't seem to delete a single message.
Is it even possible to do since I'm not trying to delete the managed object itself but another object inside it?
If not, Anyway around it?
EDIT:
Messages is an NSOrderedSet inside Conversation.
I've found this works (taken from this thread):
NSMutableOrderedSet *mutableItems = (NSMutableOrderedSet *)items.mutableCopy;
[mutableItems addObject:anItem];
items = (NSOrderedSet *)mutableItems.copy;
though I'm not sure if this is the way to go.
First, about the answer by Matt S., you are not modifying self.tempConversation so you don't have to worry about mutating the array while iterating.
On the other hand, if your problem is that self.tempConversation.chatMessages.count doesn't change. That is normal. You are deleting objects from the NSManagedObjectContext. But the array is not modified. So, the array still have the managed object BUT that managed object is deleted. Is that easy. It is a zombie managed object because it has been deleted from the MOC. Nevertheless the object has not been removed from the array. So you have a managed object with the property deleted set to YES. And it is not part of the MOC any more.
You should never, ever mutate the array you're iterating over. Per the fast enumeration docs: "It is not safe to remove, replace, or add to a mutable collection’s elements while enumerating through it. If you need to modify a collection during enumeration, you can either make a copy of the collection and enumerate using the copy or collect the information you require during the enumeration and apply the changes afterwards."
The result of mutating an array during enumeration is undefined, and my guess is core data might be just tossing up its hands and not doing anything. The reason why the mutable copy works is because you're working on a copy, not the set you're enumerating over.
I would rewrite your logic to follow the guidelines laid down in the enumeration docs, and make your changes outside of the loop.
EDIT: Additional Thoughts
Why are you locking & unlocking the persistent store? It handles that itself.
You can probably call delete safely inside the for in (but I wouldn't) and then call save outside, since save is what actually does the deletion.
More Thoughts
(Transcribing from a comment) - After thinking about this for a few days and then coming back, my guess is the reason you're not outright crashing is fast enumeration is doing a deep copy on the relationship array you're working on, because calling save on a MOC is going to increment an internal version, and then should return all existing managed objected to a faulted object to be re-fetched on next access. Really this code you've got here is actually quite dangerous from an "application health" perspective.
If you look at the documentation for core data relationships, I think you'll find the easier thing to do is just set the relationship delete rule for the relationship to "Cascade". This will remove all the messages for you when you delete the conversation. Here's the reference: https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html#//apple_ref/doc/uid/TP40001857-SW1
Say I have a CoreData entity type called Player and it has a to-one relationship (purpose) with an entity type called PlayerPurpose. For completeness, say we have an inverse relationship in PlayerPurpose called parentPlayer. Consider the following swift code:
// Assume we already have a player object in a NSManagedObjectContext called context:
player.purpose = NSEntityDescription.insertNewObjectForEntityForName("PlayerPurpose",
inManagedObjectContext: context) as PlayerPurpose;
// Later in the code, we set the value to nil (or we could have replaced
// it with another call to insertNewObjectForEntityForName)
player.purpose = nil;
// What happens to the previous playerPurpose object within the Managed Object Context?
My question: what happens to the original playerPurpose object within the Managed Object Context when the only reference it has in the data is set to nil (or replaced with another object)?
This is not really related to relationship deletion rules because I'm not explicitly deleting any object -- I'm removing it from any meaningful relationships, making it an orphan.
From an ARC perspective (if the PlayerPurpose was just a normal, non-managed object), the original PlayerPurpose instance now has no references, so it can be cleared from memory -- but what happens in the Managed Object Context? Does CoreData recognize this as an orphaned object and delete it via the context?
If not, then I assume I have to be careful to delete any managed object created via a context if I'm going to get rid of all references to it. Assuming that's the case, is there a good pattern go use for making sure that orphaned objects get cleared from the NSManagedObjectContext and that they are no longer stored in the persistent store?
Thanks!
Core Data does not automatically delete objects in this scenario, because "orphaned" is a concept that your code has but not one that Core Data recognizes. There's no reason for it to delete a PlayerPurpose object just because one of its relationships is nil.
The most reliable way to ensure that PlayerPurpose instances are deleted would be to
Create custom NSManagedObject subclasses for your entities (if you don't have them already).
Override the setter method for purpose on the Player subclass. If the new value is nil, delete the old one.
You can also handle this by just making sure to call deleteObject: at the appropriate times. Or you could run a clean-up step where you fetch every PlayerPurpose with a nil value for parentPlayer and delete them.
Problem: Need Unique Identifier
I'm saving a custom object using NSArchiver. It retains all of my objects data, however, everytime I archive and unarchive it gives them new addresses
"<Item: 0x17005d070>",
"<Item: 0x17005e4b0>",
"<Item: 0x17005e4e0>"
"<Item: 0x170059fe0>",
"<Item: 0x170059ec0>",
"<Item: 0x17005a0a0>"
For the same 3 objects.
This causes problems because I need to hold a copy of some of the items and persist this copy, and later compare the copy to the original for equality [currentItem isEqual:oldItem]; Even when this should return true (i.e. the object is the same in terms of name, location, etc.) it will return false because the pointers are different. What's the solution to this? I've thought about adding a uniqueID to each object and then just storing that uniqueID, but that seems like overkill for what I'm trying to do.
Solution:
You can't rely on an objects memory address as a unique identifier. Use NSUUID.
The problem is that you have not implemented isEqual: for your Item class. Implement it. Otherwise, as you've observed, we fall back on NSObject's definition of isEqual:, which is identicality (i.e. these are one and the same object). It is up to you to tell Cocoa that an Item should return true if the object is the same in terms of name, location, etc.; it doesn't magically know this.
The doc says:
If the object is not registered in the context, it may be fetched or
returned as a fault. This method always returns an object. The data in
the persistent store represented by objectID is assumed to exist—if it
does not, the returned object throws an exception when you access any
property (that is, when the fault is fired). The benefit of this
behavior is that it allows you to create and use faults, then create
the underlying data later or in a separate context.
I'm thinking about the last sentence:
The benefit of this behavior is that it allows you to create and use faults, then create the underlying data later or in a separate context.
Does it mean I can use objectWithID: with an arbitrary ID to get a fault handle of an non-existing object first then later create the object with ID? But how can I assign an arbitrary ID to the new object?
In general, Yes you can get a handle to a non existing item an later create that item.
But, since you don't know what ID will be assigned to the item these is not very useful in that case.
You could use obtainPermanentIDsForObjects:error: to obtain the object final ID, but, this is a trip to the store, and will have a performance penalty.
You can use objectWithID: to "warm up" the coordinator cache. in this manner you may fetch objects in the background, and use this method in another context then access these items without hitting the store (much better performance).
Since every NSManagedObjectID must initially come from a fulfilled NSManagedObject and there is no way to create one from scratch, the only possible way to "create the underlying data later" is meaningless, as follows:
NSManagedObjectID *objID = object.objectID;
[moc deleteObject:object];
…
object = [moc objectWithID:objID]; // Deleted so non-existing
[moc insertObject:object]; // Kinda of resurrecting the deleted object, but not really since the data are gone only ID is left. So it is creating a new object with the old ID. But what's the point?
// Fill data into object
…
[moc save:NULL];
If you use -objectWithID:, it will return a fault if the object is not already registered in the managed object context (ie. only if the object hasn't already been fetched and hasn't been faulted in). In the case that it does return a fault, you do not need to do anything to "create the object". Simply accessing the attributes of the object will automatically fire the fault and let you access its data. There is no additional work needed on your part to create additional objects.