Question

我正试图从NSPredicate获得一个独特的结果。

我的代码：

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Members" inManagedObjectContext:context];
    request.entity = entity;
    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"city"
                                                                                     ascending:YES
                                                                                      selector:@selector(caseInsensitiveCompare:)]];

    request.predicate = [NSPredicate predicateWithFormat:@"memberDeleted == %@", [NSNumber numberWithBool:NO]];

    NSDictionary *properties = [entity propertiesByName];
    request.propertiesToFetch = [NSArray arrayWithObject:[properties objectForKey:@"city"]];
    request.returnsDistinctResults = YES;

    request.fetchBatchSize = 20;

    NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                          managedObjectContext:context
                                                                            sectionNameKeyPath:nil
                                                                                     cacheName:@"CityCache"];
    [request release];

    self.fetchedResultsController = frc;
    [frc release];

问题是结果会在同一个城市返回很多次。该实体有很多会员，每个会员都有一个属性“city”。

我做错了什么？

谢谢，

RL

Answer 1

务必将resultType的{{1}}设置为NSFetchRequest。默认设置是返回实际对象，因此它将忽略NSDictionaryResultType。

Answer 2

在@Alex的帮助下，这里是最终代码，没有FRC：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Members" inManagedObjectContext:self.managedObjectContext];
request.entity = entity;
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"city"
                                                                                 ascending:YES
                                                                                  selector:@selector(caseInsensitiveCompare:)]];

request.predicate = [NSPredicate predicateWithFormat:@"memberDeleted == %@", [NSNumber numberWithBool:NO]];

NSDictionary *properties = [entity propertiesByName];
request.propertiesToFetch = [NSArray arrayWithObject:[properties objectForKey:@"city"]];
request.returnsDistinctResults = YES;
request.resultType = NSDictionaryResultType;
request.fetchBatchSize = 20;

NSError *error = nil;
NSArray *tempArray = [self.managedObjectContext executeFetchRequest:request error:&error];

NSMutableArray *cities = [[NSMutableArray alloc] init];
for (int i=0; i < [tempArray count]; i++){
    NSDictionary *tempDict = [NSDictionary dictionary];
    tempDict = [tempArray objectAtIndex:i];

    if ([tempDict objectForKey:@"city"] != nil)
        [cities addObject:[tempDict objectForKey:@"city"]];
}

//if the tempArray has no nil values, it's more efficient with:
//NSArray* cities = [tempArray valueForKeyPath:@"city"];

self.cityArray = cities;
[cities release];
[request release];

这将返回带有城市列表的NSArray。

感谢。

Answer 3

来自官方文件：

NSManagedObjectContext *context = <#Get the context#>;

NSEntityDescription *entity = [NSEntityDescription  entityForName:@"<#Entity name#>" inManagedObjectContext:context];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setResultType:NSDictionaryResultType];
[request setReturnsDistinctResults:YES];
[request setPropertiesToFetch :[NSArray arrayWithObject:@"<#Attribute name#>"]];

// Execute the fetch.
NSError *error;
id requestedValue = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}

具有不同属性的NSFetchRequest

3 个答案: