Question

I’m developing an application through Core Data and I need to perform some calculation in a background thread to create an xml file based on specific NSManagedObject.

Following the documentation, I set up NSOperation subclass. This class has a property like the following:

@property (nonatomic, retain) NSArray* objectIDs;

where objectIDs is an array of managed object ids (of type NSManagedObjectID). This is necessary according to the documentation: NSManagedObject are not thread safe.

Inside the main of NSOperation subclass I’m doing the following:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

NSManagedObjectContext *exportContext = [[NSManagedObjectContext alloc] init];
[exportContext setPersistentStoreCoordinator:[self persistentStoreCoordinator]];

for (NSManagedObjectID* objectID in self.orderObjectIDs) {

    NSError* error = nil;
    Order* order = (Order*)[exportContext existingObjectWithID:objectID error:&error];

    // order.someRelationship is fault here...

    // Create XML file here...
}

[exportContext reset];
[exportContext release], exportContext = nil;

[[NSNotificationCenter defaultCenter] postNotificationName:kRegisterComplete object:self];

[pool drain];
pool = nil;

Inside the for loop I’m fetching the right object using existingObjectWithID:error method of NSManagedObjectContext class since

Unlike objectWithID:, this method never returns a fault.

The method works. I’m able to retrieve the properties of that retrieved object. The only problem is that relationships are fetched as faults.

Said this, I have two questions.

First, is this the right approach to fecth NSManagedObject in a background thread?
Then, how can I fetch relationships for each fetched object within the for loop? Do I have to create a NSFetchedRequest to fetch the relationship object based on the specific object that has been fetched through the id?

Thank you in advance.

Answer 1

Why do you care that the relationship is a fault? Accessing it will fill the fault and return the data, are you concerned about disk I/O for some reason?

You may try using prefetching to alleviate some of the I/O overhead, but it’s utility is limited to the relationship on the entity being fetched:

https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html

In that case you would create a fetch request with a predicate like so:

[NSPredicate predicateWithFormat:@"SELF == %@", objectID];

That will return to you only the managed object you desire, and with relationship pre-fetching the relationship won’t be a fault.

You could further optimize this by using a IN clause and fetching all the objects at once:

[NSPredicate predicateWithFormat:@"SELF IN (%@)", desiredObjectIDs];