How to do an fetch request with expressions like this on the iPhone?

Question

The documentation has an example on how to retrieve simple values only, rather than managed objects. This remembers a lot SQL using aliases and functions to only retrieve calculated values. So, actually pretty geeky stuff. To get the minimum date from a bunch of records, this is used on the mac:

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 minimum value at the key path 'creationDate'
NSExpression *minExpression = [NSExpression expressionForFunction:@"min:" 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:@"minDate"];
[expressionDescription setExpression:minExpression];
[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;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}
else {
    if ([objects count] > 0) {
        NSLog(@"Minimum date: %@", [[objects objectAtIndex:0] valueForKey:@"minDate"];
    }
}

[expressionDescription release];
[request release];

Nice, I though - but having a deep look into NSExpression -expressionForFunction:arguments: it turns out that iPhone OS does NOT support the min: function.

Well, probably there's a nifty way to use an own function for this kind of stuff on the iPhone as well? Because on thing I'm already worrying about is, how I'm gonna sort a table based on the calculated distance of targets on a map (location-based stuff).

Answer 1

You can implement your own function, then use

+ (NSExpression *)expressionForFunction:(NSExpression *)target selectorName:(NSString *)name arguments:(NSArray *)parameters

this returns an expression which will return the result of invoking on a given target a selector with a given name using given arguments.