Swift 4 核心数据 - 获取关系 [英] Swift 4 Core Data - Fetching Relationships

问题描述

我有这样的实体:

实体1

实体2

我是这样保存数据的:

@IBAction func saveButton(_ sender: UIButton) {
    print("save")
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
        return
    }
    let context = appDelegate.persistentContainer.viewContext
    let workout = Workout(context: context)
    workout.name = workoutName.text
    workout.noOfSets = Int16(setsStepper.value)

    for index in 0..<setsVal {
        let sets = Sets(context: context)
        let test = IndexPath(row: index, section: 0)
        let cell = tableView.cellForRow(at: test) as! RepsTableViewCell
        sets.repAmount = Int16(cell.repsStepper.value)
        // Does this line not create the relationship between Workout and Set Entities?
        workout.addToSets(sets)
    }
    try! context.save()
}

我正在这样获取数据:

func fetch() {
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
        return
    }
    let context = appDelegate.persistentContainer.viewContext
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Workout")
    request.relationshipKeyPathsForPrefetching = ["Sets"]

    do {
        let result = try context.fetch(request)
        for data in result as! [NSManagedObject] {
            print(data.value(forKey: "name") as! String)
            print(data.value(forKey: "noOfSets") as! Int16)
        }
    }
    catch {
        print("failed")
    }
}

我已将实体 Workout 和 Sets 之间的关系设置为一对多，但无法从 Workout 中检索 Set 的属性.

I've set up the relationship between entities Workout and Sets as one-many, yet cannot retrieve Set's attributes from Workout.

如何检索实体关系属性?

How can I retrieve an entities relationship attributes?

尽管在 xcdatamodel 文件中设置了关系，我是否需要以编程方式指定关系?

Do I need to specify the relationship programmatically despite the relationship being setup in the xcdatamodel file?

Workout.addToSets(sets) 是否创建实体之间的关系?

Does Workout.addToSets(sets) create the relationship between the entities?

推荐答案

您应该能够以属性的形式访问您的关系实体，例如

You should be able to access your relationship entities as an attribute so for instance

for workout in result as! [Workout] {
    print(workout.name)
    if let sets = workout.sets { //assuming you have name your to-many relationship 'sets'
        print(sets.count)
    }
}

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