如何在 Swift 的 Firebase 中写入 nil 值？

Question

我有一个 struct 持有不同类型的数据，我想将其写入 Firebase。我的结构的某些属性可以为零，因此它们应该写入数据库。

写入 Firebase 时如何处理 nil 值？

 struct Booking{

  let bookingNumber:String
  let bookingCompleted:Bool
  let cancelledBy:[String:AnyObject]?

       init(bookingNumber:String,
          bookingCompleted:String){
         self.bookingNumber = bookingNumber
         self.bookingCompleted = bookingCompleted
        }

      init(cancelledBy:[String:AnyObject]){
           self.cancelledBy = cancelledBy
        }
}

class DisbursePayment {

   override func viewDidLoad() {
    super.viewDidLoad()
   //how to handle if cancelledBy is nil? It will throw error :  unexpectedly 
   //found nil while unwrapping an Optional value
   //How should I structure my data in `struct Booking`?
   let item = Booking(cancelledBy: valueCouldBeNil)  
  }
}

Answer 1

从cmets看来你已经想出了一个解决方案，我想补充以下信息以供其他人参考：

如果不需要某个值，则不应将 nil 值存储在 firebase（或任何数据库中，因为它会破坏 1NF 重复值），如果存在，则仅应将其设置为数据库。否则将其初始化为 nil 并使用可选链安全地解开该值（如果存在）。

考虑以下将其非必需属性“title”初始化为 nil 的对象

Class CustomObject: NSObject {
     //…
     var title: String? = nil
     //…
}

设置

在firebase中设置值时，如果存在，我们可以使用可选链来设置值，否则，不要将其添加到要保存到firebase中子对象引用的值中，如果存在则删除当前值。

   //Create a new reference to Firebase Database
   var ref: DatabaseReference!
   ref = Database.database().reference().child(<#your child id#>)

    //Make a Dictionary of values to set
    var values:[String:Any] = [:]

    //Set required values like uuid or other primary key

    //Use Optional Chaining to set the value
    //Note that if title is nil 
    //it doesn’t override any existing value in firebase for title i.e. the old value will still remain.
    if let title = myObject.title {
            values["title"] = title
          } else {
            //if the value was previously set but now is not, 
            //we should update firebase by removing the value.
            ref.child("title").removeValue()
         }

    //…

    //Finally, push the remaining values to firebase to up date the child
    ref.updateChildValues(values)

获取

为了从firebase获取值，我们再次使用可选链来查看给定键的值是否存在，这里我通过其子路径访问对象，您的查询可能不同但概念相同。

    //Create a new reference to Firebase Database
    var ref: DatabaseReference!
    ref = Database.database().reference().child(<# child path #>)
    ref.queryOrderedByValue().observeSingleEvent(of: .value) { (snapshot) in
            if (snapshot.value is NSNull) {
                print("No Items to Fetch")
            } else  {
                //enumerate over the Objects
                for child in snapshot.children.allObjects as! [DataSnapshot] {
                    if let object = child.value as? [String : AnyObject] {

                           let myObject = CustomObject()

                            if let title = object["title"] as? String {
                                   myObject.title = title
                              }
                               //If There is no value for 'title' it will not be set.
                               //…


//Then use the value as you normally would…
if (myObject.title != nil) {//..}