CODEWITHSUNDEEP
ioscore-data
an0
an0

Reputation: 17530

About the benefit of objectWithID:

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?

Upvotes: 1

Views: 393

Answers (3)

indragie
indragie

Reputation: 18122

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.

Upvotes: -1

an0
an0

Reputation: 17530

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];

Upvotes: 0

Dan Shelly
Dan Shelly

Reputation: 6011

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).

Upvotes: 2

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