Core data max value for property of entities children - core-data

Question

I know I shouldn't look at Core Data as something relational. But with a SQL query it is easiest to explain what I'm trying to do.
What I'd like to do is get the max value from a property constrained to the children of an entity. In SQL something like: select max(valueY) from graphdata where parentID = 10
I'm doing something akin to the above SQL query successfully excluding the where (NSPredicate) part.
I added the NSPredicate to my code thinking it would exclude the graphdata objects not within the KPI I'm looking at. Well, guess not, it's not what I'm seeing, it seems to do nothing at all really.
The Core Data Model consistst of multiple KPI objects, each containing an Array of GraphData objects. Every GraphData object contains a valueX and a valueY property.
I want the max valueY for the graphdata of a specified KPI object.
Here's my code, it's based on one of Apple's core data snippets:
NSManagedObjectContext *context = ...
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:GRAPH_DATA_ENTITY_NAME 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:#"ValueY"];
//MY WHERE KIND OF CLAUSE/PREDICATE WHICH ISN'T WORKING?
NSPredicate *predicate =
[NSPredicate predicateWithFormat:#"kpiForGraphData == %#", self.kpi];
[request setPredicate:predicate];
// Create an expression to represent the function you want to apply
NSExpression *expression = [NSExpression expressionForFunction:#"max:"
arguments:[NSArray arrayWithObject:keyPathExpression]];
// Create an expression description using the minExpression 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:#"maxValueY"];
[expressionDescription setExpression:expression];
[expressionDescription setExpressionResultType:NSDecimalAttributeType]; // For example, 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;
id requestedValue = nil;
NSArray *objects = [context executeFetchRequest:request error:&error];
if (objects == nil) {
// Handle the error.
}
else {
if ([objects count] > 0) {
requestedValue = [[objects objectAtIndex:0] valueForKey:#"maxValueY"];
}
}
[expressionDescription release];
[request release];
return [requestedValue doubleValue];
So far I've been trying to help myself by reading docs, searching through Google and stackoverflow. Everything I seem to find either selects entities (NSPredicates) or selects values using a function. Never have I seen an example or an explanation doing what I'd like to do. I like Core Data, but right now SQL seems way more straightforward to me.