Single integer as key in firebase (Firebase array behavior)

Question

If I insert data into node/a/0 in firebase.

the result will treat a is an array a[0].

The same way, if I set my data in

Answer 1

Yes, storing 0 and 1 as the key will have an issue as Firebase will think it is an array.

My simple workaround is using:

String(format:"%03d", intValue)

So that the resulting key will be "000" and "001", and they can be converted back to Int with ease.

Answer 2

TL;DR;

Workaround for REST API: add a meaningless filter like this one orderBy="$.key"&startAt="0" which actually filters-out all items with negative key): https://workaround-arrays-bagohack.firebaseio.com/matchesHeuristic.json?orderBy=%22$key%22&startAt=%220%22&print=pretty

Explanation

This is a known and unfortunate (IMHO) behaviour of Firebase, documented deep down in their support knowledge base. Quote from Best Practices: Arrays in Firebase:

Firebase has no native support for arrays. If you store an array, it really gets stored as an "object" with integers as the key names.

// we send this ['hello', 'world'] // Firebase stores this {0:'hello', 1: 'world'}

However, to help people that are storing arrays in Firebase, when you call .val() or use the REST api to read data, if the data looks like an array, Firebase will render it as an array.

In particular, *if all of the keys are integers, and more than half of the keys between 0 and the maximum key in the object have non-empty values, then Firebase will render it as an array. This latter part is important to keep in mind.

You can't currently change or prevent this behavior. Hopefully understanding it will make it easier to see what one can and can't do when storing array-like data.

So I've set up a small repro for you. Original data:

{
  "matchesHeuristic": {
      "1": {
          "id": "foo",
          "value": "bar"
      },
      "2": {
          "id": "w",
          "value": "tf"
      }
  },
  "notMatchesHeuristic": {
      "1": {
          "id": "foo",
          "value": "bar"
      },
      "365": {
          "id": "w",
          "value": "tf"
      }
  }
}

As returned by Firebase REST API: https://workaround-arrays-bagohack.firebaseio.com/.json?print=pretty

{
  "matchesHeuristic" : [ null, {
    "id" : "foo",
    "value" : "bar"
  }, {
    "id" : "w",
    "value" : "tf"
  } ],
  "notMatchesHeuristic" : {
    "1" : {
      "id" : "foo",
      "value" : "bar"
    },
    "365" : {
      "id" : "w",
      "value" : "tf"
    }
  }
}

as you can see matchesHeuristic object is transformed into an array with a null value at index 0 (because it matches the heuristic defined in Firebase docs) whereas notMatchesHeuristic is left intact. This is especially "nice" if you have dynamic data like we do - so we don't know untill runtime if it will match heauristic or not.

Workaround (REST API)

However this portion of the docs doesn't seem to hold:

You can't currently change or prevent this behavior. Hopefully understanding it will make it easier to see what one can and can't do when storing array-like data.

You can actually workaround this by requesting items searched by key, so

• this https://workaround-arrays-bagohack.firebaseio.com/matchesHeuristic.json?print=pretty is broken
• this is intact https://workaround-arrays-bagohack.firebaseio.com/matchesHeuristic.json?orderBy=%22$key%22&startAt=%220%22&print=pretty (add a meaningless filter like this one orderBy="$.key"&startAt="0" which actually filters-out all items with negative key):

{"1":{"id":"foo","value":"bar"},"2":{"id":"w","value":"tf"}}

NB: interestingly, seems Firebase support guys don't know about thi workaround (at least they didn't suggest it when we asked them about this behavior).

Answer 3

Firebase has no native support for arrays. If you store an array, it really gets stored as an "object" with integers as the key names.

However, to help people that are storing arrays in Firebase, when you call .val() or use the REST api to read data from Firebase, if the data looks like an array, Firebase will render it as an array.

In particular, if all of the keys are integers, and more than half of the keys between 0 and the maximum key in the object have non-empty values, then Firebase will render it as an array. It's this latter part of the heuristic that you are running into. Your second example only sets a value for 2, not 0 and 1, so less than half of the keys have values, and therefore Firebase renders it as an object.

You can't currently change or prevent this behavior (though it's a common source of confusion so we'll likely make some tweaks here in the future). However it's usually very easy to deal with the behavior once you understand it. If you need further help, perhaps you can expand your question to explain what you need to accomplish and how this behavior is preventing it.

Answer 4

I've encountered the same problem, but actually wanted to have a numeric key array in Swift ([Int:AnyObject]). I've written this function to make sure to always have an array (without null values):

func forceArray(from: Any?) -> [Int:AnyObject] {        
    var returnArray = [Int:AnyObject]()

    if let array = from as? [String:AnyObject] {
        for (key, value) in array {
            if let key = Int(key) {
                returnArray[key] = value
            }
        }
        return returnArray
    }

    if let array = from as? [AnyObject] {
        for (key, value) in array.enumerated() {
            if !(value is NSNull) {
                returnArray[key] = value
            }
        }
        return returnArray
    }

    return returnArray
}

Result:

["0":1, "1":2] becomes: [0:1, 1:2]

{"0":1, "6":2} becomes: [0:1, 6:2]

["0":1, "1": null, "2":2] becomes: [0:1, 2:2]

Hope this is helpful for someone!