Calculating the max value for a core data attribute - NSCFNumber error

Question

I need to find out the maximum value of an attribute of a core data entity.

I\'m still firmly in the Cocoa learning curve, and this is a simple test app that I\'m us

Answer 1

This code from Core Data Programming Guide should work.

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:context];
[request setEntity:entity];

// Specify that the request should return dictionaries.
[request setResultType:NSDictionaryResultType];

// Create an expression for the key path.
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"creationDate"];

// Create an expression to represent the maximum value at the key path 'creationDate'
NSExpression *maxExpression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:keyPathExpression]];

// Create an expression description using the maxExpression and returning a date.
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

// The name is the key that will be used in the dictionary for the return value.
[expressionDescription setName:@"maxDate"];
[expressionDescription setExpression:maxExpression];
[expressionDescription setExpressionResultType:NSDateAttributeType];

// Set the request's properties to fetch just the property represented by the expressions.
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

// Execute the fetch.
NSError *error = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}
else {
    if ([objects count] > 0) {
        NSLog(@"Maximum date: %@", [[objects objectAtIndex:0] valueForKey:@"maxDate"]);
    }
}

Answer 2

I had to solve a similar problem and approached it slightly differently by:

• querying the entity
• ordering by the attribute you want
• limit the fetch results to 1

The code for this is below. I am querying an entity called 'entityName' and retrieving the max value for the attribute 'sequenceId'.

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *res = [NSEntityDescription entityForName:@"entityName" inManagedObjectContext:managedObjectContext];
[request setEntity:res];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"sequenceId" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];

[request setFetchLimit:1];

NSError *error = nil;
NSArray *results = [managedObjectContext executeFetchRequest:request error:&error];
[request release];
if (results == nil) {
    NSLog(@"error fetching the results: %@",error);
}

NSInteger maximumValue = 0;
if (results.count == 1) {
    Result *result = (Result*)[results objectAtIndex:0];
    maximumValue =  [result.sequenceId integerValue];
}

Answer 3

The issue was that this line is wrong:

[fetch setResultType:NSDictionaryResultType];  

The result type should have been NSManagedObjectResultType

Regards

Darren.