實際上,你可以問SQL的只是價值,而不是用該值對象:
NSExpression *date = [NSExpression expressionForKeyPath:@"date"];
NSExpression *maxDate = [NSExpression expressionForFunction:@"max:"
arguments:[NSArray arrayWithObject:maxDate]];
NSExpressionDescription *d = [[[NSExpressionDescription alloc] init] autorelease];
[d setName:@"maxDate"];
[d setExpression:maxSalaryExpression];
[d setExpressionResultType:NSDateAttributeType];
[request setPropertiesToFetch:[NSArray arrayWithObject:d]];
NSError *error = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
// Handle the error.
} else {
if (0 < [objects count]) {
NSLog(@"Maximum date: %@", [[objects objectAtIndex:0] valueForKey:@"maxDate"]);
}
}
這更詳細下描述取Web管理對象 - >抓取特定值在CoreData文檔。