Query multiple keys on Firebase

Question

I'm following Firebase's recommendation of flattening data, but I'm having trouble listing a series of items from my database.

Here's a sample of my database file:

"users" : {
    "UID12349USER" : {
      "firstName" : "Jon",
      "lastName" : "Snow",
      "email" : "jonsnow@winterfell.com",
      "albums" : {
        "UID124ALBUM" : true,
        "UID125ALBUM" : true
      }
    }
},
"albums" : {
    "UID124ALBUM" : {
      "name" : "My Artwork",
    },
    "UID125ALBUM" : {
      "name" : "My Sketches",
    }
}

I'm retrieving the list of albums for a given user:

let userAlbums = database.child(usersKey).child(user.uid).child(albumsKey)
userAlbums.observeSingleEventOfType(.Value, withBlock: { snapshot in
    // fetch [UID124ALBUM: 1, UID125ALBUM: 1]
})

Now I wish I could retrieve all the user's albums in one single query. I could do a batch of queries, and populate an asynchronous array, but that doesn't seem like a good approach to me...

for key in albumKeys {
    let album = database.child(self.albumsKey).child(key)
    album.observeSingleEventOfType(.Value, withBlock: { snapshot in
        // fetch album.. append to array
    })
}

Using that approach makes it tricky to detect when the queries have finished, due to the asynchronous nature of the requests. Add to that the fact that some of the requests might fail, due to a bad connection.

Also, if I want to filter one of the albums with a given name (e.g. "My Artwork") or return nil if it doesn't exist, I also end up with a tricky end condition.

var found = false

for key in albumKeys {
    let album = database.child(self.albumsKey).child(key)
    album.observeSingleEventOfType(.Value, withBlock: { snapshot in
        // if current.name == "My Artwork"
        // completion(current)
    })
}
// This block will be called before observeSingleEventOfType =.=
if !found {
    completion(nil)
}

I have a good background on iOS and Swift, but I'm knew to Firebase and NoSQL databases. Can someone point me a good direction? Should I ditch Firebase and try something else? Am I missing some method that can query what I need? Is my json structure wrong and missing some extra keys?

Thanks

"Add to that the fact that some of the requests might fail, due to a bad connection" all requests go over the same connection, which is automatically maintained by the Firebase SDK. For a bit more on this, see stackoverflow.com/questions/35931526/… — Frank van Puffelen
That's nice, at least one thing is not a concern. Still, doesn't change the fact that there must be a better way to filter data. Given Firebase's example for Group chats, in the Guides, where one user can be part of multiple chatrooms, and one chatroom can have multiple members. How does one get (lists) all chatrooms for a given user, since all you have access in the users object is a list of chatroom ids? Thanks — Gui Moura
You'd typically keep indexes in both groups and users. So for each group, you keep a list of the users in that group. And for each user, you keep a list of the groups they're in. See firebase.google.com/docs/database/ios/structure-data#fanout — Frank van Puffelen
@FrankvanPuffelen, yeah, but with that data, how do you perform a query with the list of group ids? Let's say I'm an user with groups id1, id2, id3. How can I use this list of ids to fetch the actual groups information (name, date, etc). Thanks for the help, I'm really stuck at this, if you think it's more efficient, you can skype me at guime84 — Gui Moura
@FrankvanPuffelen, fanout vs loop through, what do you suggest if you were developing the same feature described by Guilherme? I'm not satisfied with data duplication in fanout. — Mohammad Zaid Pathan

hackerman58888 hackerman58888 · Accepted Answer · 2018-12-06T20:14:00

I would suggest using a DispatchGroup and mutual exclusion to handle asynchronous functions within a for loop. Here is the code you provided with a DispatchGroup to ensure that all of the asynchronous functions in the loop have completed before it checks the if statement:

let myGroup = DispatchGroup()
var found = false
// iterate through your array
for key in albumKeys {
    let album = database.child(self.albumsKey).child(key)
    // lock the group 
    myGroup.enter()
    album.observeSingleEventOfType(.Value, withBlock: { snapshot in
        if current.name == "My Artwork" {
             found = true
        }
        // after the async work has been completed, unlock the group
        myGroup.leave()
    })
}
// This block will be called after the final myGroup.leave() of the looped async functions complete
myGroup.notify(queue: .main) {
    if !found {
        completion(nil)
    }
}

Anything contained in the myGroup.notify(queue: .main) { codeblock will not execute until myGroup.enter() and myGroup.leave() have been called the same amount of times. Be sure to call myGroup.leave() within the Firebase observe block (after the async work) and make sure that it is called even if an error is produced from the observe.

Query multiple keys on Firebase

3 Answers