Firebase,Swift: Retrieving only those `autoID` node's which have a specific key

问题

Question--->

Is there a way to retrieve a node whose key is an autoID, which Might contain a particular key. Like in below JSON structure i have a child node powers inside some autoID's, not all of them, I want to retrieve only those nodes which have key powers in them not knowing what corresponding value might be .
In the below two suggested approach which one would consume less BandWidth?

My JSON Tree

node1   
  -node12  
    -autoId1  

        expo: "5122223333"
        users:   
            -MqrvHRBTRcPzrvAOdkklBzeFW7E2  
                firstName: "Margery"  
                lastName: "Lady" 
    -autoId2  
        powers: "Triple3"
        expo: "2123338983"
        users:   
            -LqrsadaDs12BTRcPzrvABzeFW7E2  
                firstName: "Tyrion"  
                lastName: "Imph"
  -node21  
    -autoId3  
        powers: "Triple"  
        expo: "5123333"
        users:   
            -MqrvHRBTRcPzrvAOdkklBzeFW7E2  
                firstName: "Cersie"  
                lastName: "Lady" 
    -autoId4  
        powers: "Quad"  
        expo: "2128983"
        users:   
            -LqrsadaDs12BTRcPzrvABzeFW7E2  
                firstName: "Sansa"  
                lastName: "Lady"

What Have I Tried--->

Retrieve the entire node12 and then checking which of the autoId's have a particular key. for eg lets say powers: "Triple3"

let prntRef = FIRDatabase.database().reference().child("node1").child("node12")
prntRef.observeSingleEventOfType(.Value, withBlock: {(snap) in

  if snap.exists(){

     for each in snap.value as! [String:AnyObject]{

             prntRef.child(each.0 as! String).child("powers").observeSingleEventOfType(.Value, withBlock: {(IMsnap) in

           if IMsnap.exists(){

                //Found The correct node
                }
            })
        }
      }else{

         //
        }
   })

My other alternative solution is:-

FIRDatabase.database().reference().child("node1").child("node12").queryOrderedByChild("powers").observeSingleEventOfType(.Value, withBlock: {(snap) in

   if let snapDict = snap.value! as? [String : AnyObject]{

           print(snapDict.keys.first!)   //Retrieving My AutoID .Nut this gives me entire node. 

        }

        for each in snap.value as! [String:AnyObject]{

             print(each.0)   //Retrieving My AutoID 

       }
   })

})

Note:- I found somewhat similar Q posted in Firebase forum, but no one seems to answer it :-https://groups.google.com/forum/#!topic/firebase-talk/ZDHKwxRMiKQ

回答1:

If you don't care about the value of powers, you only care that the key exists under the autoId node, you just need .queryEqualToValue("") (this is dependent on the value of the key-value pair being a string, for a number, instead of "" just use 0 [dependent on your values being greater than or equal to 0]).

let ref = FIRDatabase.database().referenceWithPath("node1/node12")

ref.queryOrderedByChild("powers")
ref.queryEqualToValue("")
ref.observeEventOfType(.Value, withBlock: { snap in

    print(snap) // all the autoId nodes that have the powers key
})

来源：https://stackoverflow.com/questions/39420001/firebase-swift-retrieving-only-those-autoid-nodes-which-have-a-specific-key

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