Question

In my first attempt at using NSExpression in a fetch request, I’m getting a result that is consistently one off from what I get if I use a regular fetch request.

The MO “Subject” has a to-many relationship to the MO “Book,” the inverse being to-one.

This is the NSExpression fetchRequest I’m using:

Project_AppDelegate *appDelegate = [[NSApplication sharedApplication] delegate];
NSManagedObjectContext *context = [appDelegate managedObjectContext];
NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@“Book”
                                                     inManagedObjectContext:context];
Subject *subjectToDelete = [self.arrayOfSubjects objectAtIndex:indexSelected];    
NSPredicate *pred = [NSPredicate predicateWithFormat:@"subject == %@", subjectToDelete];
NSExpression *expn = [NSExpression expressionForFunction:@"count:" 
                                                     arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:@"idPerm"]]];
NSExpressionDescription *expnDesc = [[NSExpressionDescription alloc] init];
[expnDesc setExpression:expn];
[expnDesc setName:@“countMatchingBooks”];
[expnDesc setExpressionResultType:NSInteger64AttributeType];
NSArray *properties = [NSArray arrayWithObject:expnDesc];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entityDescription]; 
[request setPredicate:pred];
[request setPropertiesToFetch:properties];
[request setResultType:NSDictionaryResultType];
NSError *error = nil; 
NSArray *results = [context executeFetchRequest:request error:&error];
if (error) {
    // error handling here
}
[request release];
[expnDesc release];

// Retrieve the count from the results array.
NSNumber *numBooksAssignedSubjectToDelete = [[results objectAtIndex:0] valueForKey:@“countMatchingBooks”];
uint64_t uloloBooksAssignedSubjectToDelete = [numBooksAssignedSubjectToDelete unsignedLongLongValue];

(The idea is to present the user with a confirmation panel advising them of how many Books will be deleted via the Cascade rule if they choose to delete the chosen Subject — without faulting the Book MOs at this point.)

And this is the simple fetchRequest I’m using as a test:

NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@“Book”
                                                     inManagedObjectContext:contextMain];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entityDescription]; 
NSError *error = nil; 
NSArray *booksAll = [contex executeFetchRequest:request error:&error];
[request release];
// Loop through the “booksAll” array and count those whose subject matches the one assigned as “subjectToDelete”

What happens is that if the NSExpression fetchRequest returns a count of n, the simple fetchRequest returns a count of n + 1.

Thinking the fetchRequests themselves might be somehow altering the data, I tried running them in a different order, but with the same result.

Maybe requests using expressions skip MOs which have not yet been saved? No. I ran a test that creates a bunch of new “Book” MOs to see if the gap between expression request and regular request would widen. It remained exactly one off.

Any idea what I’m doing wrong?

Answer 1

NSFetchRequests using NSExpressionDescription does not include unsaved objects. NSFetchRequest has a method -setIncludePendingChanges:, which does not accept YES when the result type is NSDictionaryResultType. This means that you cannot use NSExpressionDescription to get unsaved objects.