非重复计数

Question

我希望拥有与SQL函数等效的核心数据：

SELECT species, sex, COUNT(*) FROM Bird GROUP BY species, sex;

通常会返回以下内容的请求

+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| Bus     | m    |        2 |
| Car     | f    |        1 |
+---------+------+----------+

具有以下条目：

INSERT INTO  Bird VALUES ('BlueBird','Car','f');
INSERT INTO  Bird VALUES ('RedBird','Bus','m');
INSERT INTO  Bird VALUES ('RedBird','Bus','m');

我已经成功地完成了不同的请求，但我在计数(*)时遇到了问题。这就是我所拥有的：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Bird" inManagedObjectContext:managedObjectContext];
[request setEntity:entity]; 
[request setReturnsDistinctResults:YES];
[request setResultType:NSDictionaryResultType];
NSDictionary *entityProperties = [entity propertiesByName];

NSMutableArray *properties = [NSMutableArray arrayWithObject:[entityProperties objectForKey:@"species"]];
[properties addObject:[entityProperties objectForKey:@"sex"]];
[request setPropertiesToFetch: properties];

我应该添加什么？

非常感谢

Answer 1

样本数据所隐含的数据模型表明，您必须获取每个对象、过滤和重新获取：

NSManagedObjectContext *moc;
NSEntityDescription *birdEntity = [NSEntityDescription entityForName:"Bird" inManagedObjectContext:moc];

NSFetchRequest *fetchAllBirds = [[NSFetchRequest new] autorelease];
[fetchAllBirds setEntity:birdEntity];
NSArray *allBirds = [moc executeFetchRequest:fetchAllBirds error:NULL];

NSArray *species = [allBirds valueForKeyPath:@"@distinctUnionOfObjects.species"];

NSMutableDictionary *speciesCounts = [NSMutableDictionary dictionaryWithCapacity: [species count]];
for (NSString *speci in species)
{
    NSFetchRequest *fetchSpeci = [[NSFetchRequest new] autorelease];
    [fetchSpeci setEntity: birdEntity];
    [fetchSpeci setPredicate:[NSPredicate predicateWithFormat:@"speci == %@", speci]];

    int speciCount = [moc countForFetchRequest:fetchSpeci];
    [speciesCounts setObject:[NSNumber numberWithInt:speciCount] forKey:speci];
}

我建议对数据进行重构。您可以将species属性替换为与Species实体的关系。

CoreData不是关系数据库，它是一个对象图持久化框架。如果您试图将其用作关系数据库，您将以混乱告终。