How to do an fetch request with expressions like this on the iPhone?
- by dontWatchMyProfile
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).
