swift - NSPredicate |在 Entity Fetch Request 上给出错误的异常 | swift 4

Question

我尝试通过 CoreData 模型的 NSPredicate 获取实体。最初我使用它的 id 插入一个实体，现在打算获取它。然而，我的代码崩溃了，没有对我的 NSPredicate 做任何进一步的描述。我不知道为什么？我的 id 属性是 Int32:

        var employTime:EmployeeTime?
        let context = getContext()
        let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "EmployeeTime")

        //code crashes on this bottom line
        let predicate = NSPredicate(format: "id == %@", id)
        fetchRequest.predicate = predicate
        let employTimes = try! context.fetch(fetchRequest) as? [EmployeeTime]
        if (employTimes?.count)! > 0 {
            for et in employTimes! {
                if et.id == id {
                    employTime = et
                }
            }
        } else {
            employTime = EmployeeTime(context: context)
        }
        return employTime!

非常感谢任何帮助。

Answer 1

这是一个很常见的错误。

占位符 %@ 仅用于对象，对于 Int32 您需要 %d 或 %i

let predicate = NSPredicate(format: "id == %d", id)

并使用具体的 NSManagedObject 子类创建获取请求

let fetchRequest = NSFetchRequest<EmployeeTime>(entityName: "EmployeeTime")

这避免了以后的类型转换。

---

您的整个代码都可以简化(不需要可选):

let employTime:EmployeeTime
let context = getContext()
let fetchRequest = NSFetchRequest<EmployeeTime>(entityName: "EmployeeTime")
let predicate = NSPredicate(format: "id == %d", id)
fetchRequest.predicate = predicate
let employTimes = try! context.fetch(fetchRequest)
if let et = employTimes.first {
   employTime = et
} else {
   employTime = EmployeeTime(context: context)
}
return employTime

但是你应该避免try!。使您的函数 throw 并将潜在的错误移交给调用者。