從核心數據中篩選標題

Question

我有一個擁有標題標題和金額的實體。我想顯示我實體中每個標題的總價值。

Answer 1

您使用NSPredicate來篩選按類別的對象，然後就可以獲取某個屬性例如只值：

NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
// Entity 
NSEntityDescription *ent = [NSEntityDescription entityForName:@"Incoming" inManagedObjectContext:context]; 
[request setEntity:ent]; 
// Predicate 
NSPredicate *pred = [NSPredicate predicateWithFormat:@"category MATCHES[cd] %@", @"FOOD"]; 
[request setPredicate:pred]; 
// Property fetch 
[request setResultType:NSDictionaryResultType]; 
[request setReturnsDistinctResults:NO]; 
[request setPropertiesToFetch:@[@"Amount"]]; 

// Execute the fetch. 
NSError *error; 
NSArray *objects = [context executeFetchRequest:request error:&error]; 
int total = 0; 
if (objects) { 
    for(NSDictionary *dict in objects) { 
     NSString *key = dict.allKeys.lastObject; 
     total += [dict[key] intValue]; 
    } 
} else { 
    NSLog(@"Fetch error: %@", error.localizedDescription); 
} 
NSLog(@"Total: %i", total); 

Answer 2

下面將得到一個類別和金額讀取操作：

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Incoming"]; 
fetchRequest.resultType = NSDictionaryResultType; 

NSExpression *sumExp = [NSExpression expressionWithFormat:@"sum:(amount)"]; 
NSExpressionDescription *sumED = [[NSExpressionDescription alloc] init]; 
sumED.name = @"sum"; 
sumED.expression = sumExp; 
sumED.expressionResultType = NSDoubleAttributeType; 

fetchRequest.propertiesToFetch = [@"title", sumED]; 
fetchRequest.propertiesToGroupBy = [@"title"]; 

NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil]; 

查看NSExpression文檔here。

Answer 3

通過利用KVC（鍵值編碼），您幾乎可以免除提取請求。

首先獲取（或過濾器）與此斷言：

[NSPredicate predicateWithFormat:@"category = %@", @"Food"]; 

然後就總結得出一個結果行：

NSNumber *sum = [result valueForKeyPath:@"@sum.amount"]; 

沒有必要獲取使用NSDictionaryResultType只有某些屬性 - 只需獲取正常的NSManagedObject（核心數據將爲您優化）。