Swift中的CoreData总和

Question

我有一个具有十进制Transaction属性的CoreData实体amount。我想获取由谓词过滤的所有金额的总和。 Altohugh有解决方案using loops，我想直接在CoreData because of performance中进行（并了解NSExpression）。

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以下代码按预期工作，但是，正如我所说，这不是我想要的

// Easy way: Fetch all and use `value(forKeyPath:)`
let transactionFetch = NSFetchRequest<NSManagedObject>(entityName: CoreDateWrapper.Entity.Transaction)

var results: [NSManagedObject] = []

do {
    results = try wrapper.context.fetch(transactionFetch)
} catch {
    print("Error: \(error.localizedDescription)")
}

print((results as NSArray).value(forKeyPath: "@sum.\(CoreDateWrapper.Attribute.amount)") as! Decimal)

现在，我想使用NSExpression，但是获取结果始终是一个空数组。

// Setup the expression and expression-description
let amountExpression = NSExpression(forKeyPath: CoreDateWrapper.Attribute.amount)
let sumExpression = NSExpression(forFunction: "sum:", arguments: [amountExpression])
let sumDescription = NSExpressionDescription()
sumDescription.expression = sumExpression
sumDescription.expressionResultType = .decimalAttributeType
sumDescription.name = "sum"

// Setup the fetchRequest to only get the sum.
// I expect a dictionary as a result instead of a `NSManagedObject`
let sumFetch = NSFetchRequest<NSDictionary>(entityName: CoreDateWrapper.Entity.Transaction)
sumFetch.propertiesToFetch = [sumDescription]
sumFetch.resultType = .dictionaryResultType

// Fetch the sum
var sumResult: [String: Decimal]? = nil
do {
    let array = try wrapper.context.fetch(sumFetch)
       if let res = array.first as? [String: Decimal] {
        sumResult = res
    } else {
        print("Wrong type for result")
    }

} catch {
    print("Error fetching result: \(error.localizedDescription)")
}

// Output the sum
if let sum = sumResult?["sum"] {
    print("Total: \(sum)")
}

这总是打印错误的类型 ，因为array为空。