ios - NSFetchedResultController 与 SUM

Question

我应该如何通过第一个属性获取数据并对第二个属性的值求和？

通过简单的获取我有:

但我只需要在 1 行中显示相同的 ZINGREDIENT 并对 ZCOUNT 的值求和:

就像:

select ZINGREDIENT, sum(ZCOUNT) 
from Table
group by ZINGREDIENT

Answer 1

这是非常简单的 NSFetchRequest 版本:

    NSManagedObjectContext* context = [object managedObjectContext];
    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"Join"];
    [request setResultType:NSDictionaryResultType];
    [request setReturnsDistinctResults:YES];
    NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"count"];
    NSExpression* expr = [NSExpression expressionForFunction:@"sum:" arguments:@[keyPathExpression]];
    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
    [expressionDescription setName:@"count"];
    [expressionDescription setExpression:expr];
    [expressionDescription setExpressionResultType:NSInteger32AttributeType];
    [request setPropertiesToGroupBy:@[@"ingredient"]];
    [request setPropertiesToFetch:@[@"ingredient",expressionDescription]];
    NSError* error = nil;
    NSArray* rez = [context executeFetchRequest:request error:&error];