Question

我有一个拥有头衔和金额的实体。我想显示我实体中每个标题的总价值。

Answer 1

通过利用KVC（键值编码），您几乎可以省去获取请求。

首次使用此谓词获取（或过滤）：

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

然后将结果汇总到一行：

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

无需仅使用NSDictionaryResultType获取某些属性 - 只需获取正常的NSManagedObject（核心数据将为您优化）。

Answer 2

您使用NSPredicate按类别过滤对象，然后您只能获取某个属性的值，例如：

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 3

以下内容将在一次获取操作中获取类别和总和：

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];

请参阅NSExpression文档here。

从核心数据中筛选标题

3 个答案: