CoreData检查对多关系是否包含对象

Question

我的应用程序中具有以下数据模型：

基本上，我想存储国家名称和城市名称。每个城市属于一个国家，一个国家包含0-n个城市。

现在我将新城市添加到某个国家之前，我需要知道该国家是否已经包含具有该名称的城市。

直到现在我这样做：

- (BOOL)countryForName:(NSString *)countryName containsCity:(NSString *)cityName {
    Countries *country = [self countryForName:countryName];
    NSSet *cityNames = [country valueForKey:@"cities"];
    for (Cities *city in cityNames) {
        if ([city.cityName isEqualToString:cityName]) {
            return YES;
        }
    }
    return NO;
}

这显然很慢，我需要的是带有正确谓词的等效获取。我对这样的一个实体进行搜索：

    NSEntityDescription *entity     = [NSEntityDescription entityForName:@"Countries" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor    = [[NSSortDescriptor alloc] initWithKey:@"countryName" ascending:YES];
    NSArray *sortDescriptors            = @[sortDescriptor];
    fetchRequest.sortDescriptors        = sortDescriptors;

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"GetCountryList"];
    aFetchedResultsController.delegate  = self;
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"countryName == %@", countryName];
    [aFetchedResultsController.fetchRequest setPredicate:predicate];

但是，如果搜索涉及多个实体，该怎么办？换句话说：如何仅为一个国家/地区添加唯一的城市名称？非常感谢！

Answer 1

[如果您要检查给定的国家/地区对象是否已包含具有名称的城市，你可以做

Countries *country = ...
NSString *cityName = ...
if ([[country valueForKeyPath:@"cities.name"] containsObject:cityName]) {
    // ...
}

或者，使用获取请求：

NSString *countryName = ...
NSString *cityName = ...
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Countries"];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"countryName == %@ AND ANY cities.cityName == %@", countryName, cityName];
[fetchRequest setPredicate:predicate];

NSError *error;
NSUInteger count = [self.managedObjectContext countForFetchRequest:fetchRequest error:&error];

if (count == NSNotFound) {
    // error
} else if (count == 0) {
    // no matching country
} else {
    // at least one matching country
}

注意，NSFetchedResultsController通常用于显示在表视图中获取请求，因此此处不需要。