Compare values with an array of values with Firebase. Swift

Question

I have a value which needs to be compared with an array of values. Basically a user needs to check if it has the same item as another user. But I am struggling to solve this as how do I get the item of an array of users? Normally when you observe a value you do something like this:

Database.database().reference(withPath: "users/\(userID)/itemList").observeSingleEvent...

However, when it comes to an array of users itemList how can this be achieved if there are multiple ID's? I'd like compare a user item or items with other users item and check if they match in order to sort an array of users that have a match.

If there is already an example for this please help direct me there.

Update

This is how my data structure looks:

{
  "users": {
    "EWJGFYmVTzOiHgnq42ebhrg2fj": {
      "firstName": "Friederike",
      "itemList": [
        2: true,
        3: true,
        0: true
      ]
    },

    "C076OYmVTzOiHgnq4wPQtY2XpED2": {
      "firstName": "Ian",
      "itemList": [
        0: true,
        1: true,
        3: true
      ]
    },
    "Juoiuf0N6qNmkm32jrtu6X6UK62": {
      "itemList": [
        0: true
      ],
      "firstName": "Jack"
    }
  }
}

Update 2.0

With the answer below I am able to query through the items table but still unable to query the keys that match as there can be multiple arrays and therefore I cannot use it to filter or anything.

Answer 1

Ok so with your current data structure you'd have to query all nodes under users and then compare the arrays, which is very inefficient. There isn't a straight forward or easy way to do that without modifying your structure. So I suggest you modify your data structure so that each item has a list of all users that have it. Something like this:

{
  "items": {
    "0": {
      "EWJGFYmVTzOiHgnq42ebhrg2fj": "Friederike",
      "C076OYmVTzOiHgnq4wPQtY2XpED2": "Ian",
      "Juoiuf0N6qNmkm32jrtu6X6UK62": "Jack"
    },

    "1": {
      "C076OYmVTzOiHgnq4wPQtY2XpED2": "Ian"
    },

    "2": {
      "EWJGFYmVTzOiHgnq42ebhrg2fj": "Friederike"
    }
    //....
  }
}

Depending on what you want to display you might want to store more information than just the users UID and username. You can query all the users that have the same items as you using a query like this:

let ref = Database.database().reference()
// assuming you already have the current users items stored in an array
for item in items {
    ref.child("items").child(String(item)).observeSingleEvent(of: .value, with: { snap in
        for child in snap.children {
            let child = child as? DataSnapshot
            if let key = child?.key, let name = child?.value as? String {
                // do something with this data
            }
        }
    })
}

Firebase database is noSQL, so data is meant to be denormalized or duplicated so that queries can be optimized. Firebase actually recommends that you avoid nesting data. Take a look at this question for more information.

Hope that helps

Code related to question asked in comments

Assuming you are storing the UID's or names of users with the same items in a string array you can prevent duplicates using .contains()

var namesWithMatchingItems = [String]()

if !namesWithMatchingItems.contains(nameYouJustFetched) {
    namesWithMatchingItems.append(nameYouJustFetched)
}