重複計數

Question

我想有SQL函數的核心數據等效的：

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

請求指通常會以下輸入返回此

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

：

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

我已經完成了不同的請求，但我在計算（*）時遇到了問題。 以下是我有：

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]; 

什麼我要補充？

非常感謝

Answer 1

的數據模型的樣本數據暗示表明，你必須獲取每個對象，過濾器和重新獲取：

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]; 
} 

我建議重塑數據。您可以將species屬性替換爲與Species實體的關係。

CoreData不是一個關係數據庫，它是一個對象圖persistnts框架。如果您嘗試將其用作關係數據庫，您最終會陷入混亂。