Objective c 使用另一个表达式作为sum:expression的参数从NSManagedObject获取聚合数据

是否可以使用表达式作为

sum:

expression的参数

我有实体（

NSManagedObject

class）

Drink

，属性为

costPerDrink

，

numberOfDrinks

和

drinkingDate

我想得到一段时间内总成本的总和（

numberOfDrinks

乘以

costPerDrink

）。虽然我在获取单个属性的和（例如时间段内

numberOfDiverses

的和）时没有问题，但当我尝试在另一个（乘法）表达式上使用和表达式时，我得到了错误：

NSInvalidArgumentException, reason: Unsupported argument to sum : 
(
    "costPerDrink * numberOfDrinks" 
)

见代码：

// Init fech
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Drink" inManagedObjectContext:[Service appContext]];
[request setEntity:entity];

// Return dictionary
[request setResultType:NSDictionaryResultType];

// Set conditions
[request setPredicate:[NSPredicate predicateWithFormat:@"(drinkingDate>=%@) AND (drinkingDate<=%@)", self.startDate, self.endDate]];

// Set 1st column to be returned - count of drinks
NSExpression *drinksExpression = [NSExpression expressionForKeyPath:@"numberOfDrinks"];
NSExpression *sumDrinksExpression = [NSExpression expressionForFunction:@"sum:" arguments:[NSArray arrayWithObject:drinksExpression]];
NSExpressionDescription *sumDrinksResultDescription = [[NSExpressionDescription alloc] init];
[sumDrinksResultDescription setName:@"sumDrinks"];
[sumDrinksResultDescription setExpression:sumDrinksExpression];
[sumDrinksResultDescription setExpressionResultType:NSInteger32AttributeType];

// Set 2nd column to be returned - total cost
NSExpression *costExpression = [NSExpression expressionForKeyPath:@"costPerDrink"];
NSExpression *totalExpression = [NSExpression expressionForFunction:@"multiply:by:"
                                                            arguments:[NSArray arrayWithObjects:costExpression,
                                                                                          drinksExpression, nil]];
NSExpression *sumCostExpression = [NSExpression expressionForFunction:@"sum:" arguments:[NSArray arrayWithObject:totalExpression]];
NSExpressionDescription *sumCostResultDescription = [[NSExpressionDescription alloc] init];
[sumCostResultDescription setName:@"sumCost"];
[sumCostResultDescription setExpression:sumCostExpression];
[sumCostResultDescription setExpressionResultType:NSDecimalAttributeType];

// Add expressions to fetch
[request setPropertiesToFetch:[NSArray arrayWithObjects: sumDrinksResultDescription, sumCostResultDescription, nil]];

// Execute fech
NSError *error;
NSArray *result = [[Service appContext] executeFetchRequest:request error:&error];

//初始化fech
NSFetchRequest*request=[[NSFetchRequest alloc]init]；
NSEntityDescription*entity=[NSEntityDescription entityForName:@“Drink”inManagedObjectContext:[Service appContext]]；
[请求集合实体：实体]；
//返回字典
[请求setResultType:NSDictionaryResultType]；
//设定条件
[request setPredicate:[NSPredicate Predicate WithFormat:@”（drinkingDate>=%@）和（drinkingDate）我过去也遇到过类似的问题，我按照这里的说明手动计算了它。但是，如果有任何核心数据专家对此作出答复，我将非常高兴。