如何检查实体属性名称匹配，并重写（更新）其他属性的数据？

Question

我正在使用带有属性的Core Data实体，这里是生成的子类代码：

extension City {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<City> {
        return NSFetchRequest<City>(entityName: "City")
    }
    @NSManaged public var name: String?
    @NSManaged public var description: String?
    @NSManaged public var temp: Double
    @NSManaged public var temp_max: Double
    @NSManaged public var temp_min: Double
}

我正在解析JSON数据并通过Weather模型处理它，并使用此代码将数据保存到数据库中（它运行良好）：

func saveWeatherData() {

        let ad = UIApplication.shared.delegate as! AppDelegate
        let context = ad.persistentContainer.viewContext

        let city = City(context: context)

        city.name = Weather.locationName!

        city.description = Weather.details!

        city.temp = Weather.temp

        city.temp_min = Weather.tempMin

        city.temp_max = Weather.tempMax

        ad.saveContext()
    }

问题是......如何检查城市名称的重合（名称属性）？如果这样的城市已经存在于数据库中，而不是创建新记录，则覆盖（更新）当前属性（description，temp，temp_max，temp_min）的值？谢谢。

Answer 1

您只需要尝试获取现有对象。如果获取成功，则更新其属性。如果未找到匹配项，则创建一个新对象。就像是：

func saveWeatherData() {

    let ad = UIApplication.shared.delegate as! AppDelegate
    let context = ad.persistentContainer.viewContext

    let fetchRequest:NSFetchRequest<City> = City.fetchRequest()
    fetchRequest.predicate = NSPredicate(format:"name = %@",Weather.locationName)

    fetchRequest.fetchLimit = 1  

    do {
       let result = try context.fetch(fetchRequest)
       let city = result.first ?? City(context: context)

       city.name = Weather.locationName!
       city.description = Weather.details!
       city.temp = Weather.temp
       city.temp_min = Weather.tempMin
       city.temp_max = Weather.tempMax

       ad.saveContext()
    }
    catch {
        print("Error fetching city: \(error.localizedDescription)"
    }
}