Question

Using Core Data, I encountered a problem. I have an entity “Movement” with an attribute “amount”. How do I make the sum of all the “amount” of all instances? I’d like to understand how to use NSExpressionDescription, but it’s good enough NSSet.

Answer 1

Having a managedObjectContext:

NSManagedObjectContext *managedObjectContext = ...

We create a fetch request with return type dictionary:

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([Movement class])];
fetchRequest.resultType = NSDictionaryResultType;

Then we create an expression description to calculate the sum:

NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
expressionDescription.name = @"sumOfAmounts";
expressionDescription.expression = [NSExpression expressionForKeyPath:@"@sum.amount"];
expressionDescription.expressionResultType = NSDecimalAttributeType;

Set the request properties to fetch:

fetchRequest.propertiesToFetch = @[expressionDescription];

We could also set a predicate if we want.

And last we execute the request and get an array containing a dictionary with one key(@”sumOfAmounts”) and its value is a NSNumber with the sum of amounts.

NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (result == nil)
{
    NSLog(@"Error: %@", error);
}
else
{
    NSNumber *sumOfAmounts = [[result objectAtIndex:0] objectForKey:@"sumOfAmounts"];
}

Cheers