Fetch object by property in Core Data - iphone

Question

In my iPhone project, I want to write a function that checks wether there's an object in my Core Data ManagedObjectContext with a given value for a certain property, say some_property.
If there's already an object with some_property == 12, I want the function to return the object, otherwise, I want to create the object, or at least return nil.
How would I do that?

Answer 1

The following snippet shows how to retrieve the objects matching a specific predicate. If there are no such objects, the snippet shows how to create a new object, save it and return it.
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"YourEntityName" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
// retrive the objects with a given value for a certain property
NSPredicate *predicate = [NSPredicate predicateWithFormat: #"property == %#", value];
[request setPredicate:predicate];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"yourSortKey" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:#"Root"];
aFetchedResultsController.delegate = self;
NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:request error:&error];
[request release];
[sortDescriptor release];
[sortDescriptors release];
if ((result != nil) && ([result count]) && (error == nil)){
return [NSMutableArray arrayWithArray:result];
}
else{
YourEntityName *object = (YourEntityName *) [NSEntityDescription insertNewObjectForEntityForName:#"YourEntityName" inManagedObjectContext:self.managedObjectContext];
// setup your object attributes, for instance set its name
object.name = #"name"
// save object
NSError *error;
if (![[self managedObjectContext] save:&error]) {
// Handle error
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
return object;
}

Answer 2

It's better if you don't do multiple fetching if you want to check for certain properties on the local data. Just do one fetch request using a pre-populated array and then iterate or filter the results.
This is a code snippet from Core Data Programming Guide "Implementing Find-or-Create Efficiently":
// get the names to parse in sorted order
NSArray *employeeIDs = [[listOfIDsAsString componentsSeparatedByString:#"\n"]
sortedArrayUsingSelector: #selector(compare:)];
// create the fetch request to get all Employees matching the IDs
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:
[NSEntityDescription entityForName:#"Employee" inManagedObjectContext:aMOC]];
[fetchRequest setPredicate: [NSPredicate predicateWithFormat: #"(employeeID IN %#)", employeeIDs]];
// make sure the results are sorted as well
[fetchRequest setSortDescriptors: [NSArray arrayWithObject:
[[[NSSortDescriptor alloc] initWithKey: #"employeeID"
ascending:YES] autorelease]]];
// Execute the fetch
NSError *error;
NSArray *employeesMatchingNames = [aMOC
executeFetchRequest:fetchRequest error:&error];