Develop Reference

How to reference one Node value in Other Node in firebase Android [duplicate]

I've read the Firebase docs on Stucturing Data. Data storage is cheap, but the user's time is not. We should optimize for get operations, and write in multiple places.
So then I might store a list node and a list-index node, with some duplicated data between the two, at very least the list name.
I'm using ES6 and promises in my javascript app to handle the async flow, mainly of fetching a ref key from firebase after the first data push.
let addIndexPromise = new Promise( (resolve, reject) => {
let newRef = ref.child('list-index').push(newItem);
resolve( newRef.key()); // ignore reject() for brevity
});
addIndexPromise.then( key => {
ref.child('list').child(key).set(newItem);
});
How do I make sure the data stays in sync in all places, knowing my app runs only on the client?
For sanity check, I set a setTimeout in my promise and shut my browser before it resolved, and indeed my database was no longer consistent, with an extra index saved without a corresponding list.
Any advice?

Great question. I know of three approaches to this, which I'll list below.
I'll take a slightly different example for this, mostly because it allows me to use more concrete terms in the explanation.
Say we have a chat application, where we store two entities: messages and users. In the screen where we show the messages, we also show the name of the user. So to minimize the number of reads, we store the name of the user with each chat message too.
users
so:209103
name: "Frank van Puffelen"
location: "San Francisco, CA"
questionCount: 12
so:3648524
name: "legolandbridge"
location: "London, Prague, Barcelona"
questionCount: 4
messages
-Jabhsay3487
message: "How to write denormalized data in Firebase"
user: so:3648524
username: "legolandbridge"
-Jabhsay3591
message: "Great question."
user: so:209103
username: "Frank van Puffelen"
-Jabhsay3595
message: "I know of three approaches, which I'll list below."
user: so:209103
username: "Frank van Puffelen"
So we store the primary copy of the user's profile in the users node. In the message we store the uid (so:209103 and so:3648524) so that we can look up the user. But we also store the user's name in the messages, so that we don't have to look this up for each user when we want to display a list of messages.
So now what happens when I go to the Profile page on the chat service and change my name from "Frank van Puffelen" to just "puf".
Transactional update
Performing a transactional update is the one that probably pops to mind of most developers initially. We always want the username in messages to match the name in the corresponding profile.
Using multipath writes (added on 20150925)
Since Firebase 2.3 (for JavaScript) and 2.4 (for Android and iOS), you can achieve atomic updates quite easily by using a single multi-path update:
function renameUser(ref, uid, name) {
var updates = {}; // all paths to be updated and their new values
updates['users/'+uid+'/name'] = name;
var query = ref.child('messages').orderByChild('user').equalTo(uid);
query.once('value', function(snapshot) {
snapshot.forEach(function(messageSnapshot) {
updates['messages/'+messageSnapshot.key()+'/username'] = name;
})
ref.update(updates);
});
}
This will send a single update command to Firebase that updates the user's name in their profile and in each message.
Previous atomic approach
So when the user change's the name in their profile:
var ref = new Firebase('https://mychat.firebaseio.com/');
var uid = "so:209103";
var nameInProfileRef = ref.child('users').child(uid).child('name');
nameInProfileRef.transaction(function(currentName) {
return "puf";
}, function(error, committed, snapshot) {
if (error) {
console.log('Transaction failed abnormally!', error);
} else if (!committed) {
console.log('Transaction aborted by our code.');
} else {
console.log('Name updated in profile, now update it in the messages');
var query = ref.child('messages').orderByChild('user').equalTo(uid);
query.on('child_added', function(messageSnapshot) {
messageSnapshot.ref().update({ username: "puf" });
});
}
console.log("Wilma's data: ", snapshot.val());
}, false /* don't apply the change locally */);
Pretty involved and the astute reader will notice that I cheat in the handling of the messages. First cheat is that I never call off for the listener, but I also don't use a transaction.
If we want to securely do this type of operation from the client, we'd need:
security rules that ensure the names in both places match. But the rules need to allow enough flexibility for them to temporarily be different while we're changing the name. So this turns into a pretty painful two-phase commit scheme.
change all username fields for messages by so:209103 to null (some magic value)
change the name of user so:209103 to 'puf'
change the username in every message by so:209103 that is null to puf.
that query requires an and of two conditions, which Firebase queries don't support. So we'll end up with an extra property uid_plus_name (with value so:209103_puf) that we can query on.
client-side code that handles all these transitions transactionally.
This type of approach makes my head hurt. And usually that means that I'm doing something wrong. But even if it's the right approach, with a head that hurts I'm way more likely to make coding mistakes. So I prefer to look for a simpler solution.
Eventual consistency
Update (20150925): Firebase released a feature to allow atomic writes to multiple paths. This works similar to approach below, but with a single command. See the updated section above to read how this works.
The second approach depends on splitting the user action ("I want to change my name to 'puf'") from the implications of that action ("We need to update the name in profile so:209103 and in every message that has user = so:209103).
I'd handle the rename in a script that we run on a server. The main method would be something like this:
function renameUser(ref, uid, name) {
ref.child('users').child(uid).update({ name: name });
var query = ref.child('messages').orderByChild('user').equalTo(uid);
query.once('value', function(snapshot) {
snapshot.forEach(function(messageSnapshot) {
messageSnapshot.update({ username: name });
})
});
}
Once again I take a few shortcuts here, such as using once('value' (which is in general a bad idea for optimal performance with Firebase). But overall the approach is simpler, at the cost of not having all data completely updated at the same time. But eventually the messages will all be updated to match the new value.
Not caring
The third approach is the simplest of all: in many cases you don't really have to update the duplicated data at all. In the example we've used here, you could say that each message recorded the name as I used it at that time. I didn't change my name until just now, so it makes sense that older messages show the name I used at that time. This applies in many cases where the secondary data is transactional in nature. It doesn't apply everywhere of course, but where it applies "not caring" is the simplest approach of all.
Summary
While the above are just broad descriptions of how you could solve this problem and they are definitely not complete, I find that each time I need to fan out duplicate data it comes back to one of these basic approaches.

To add to Franks great reply, I implemented the eventual consistency approach with a set of Firebase Cloud Functions. The functions get triggered whenever a primary value (eg. users name) gets changed, and then propagate the changes to the denormalized fields.
It is not as fast as a transaction, but for many cases it does not need to be.

Firebase Database data structure with Firebase Authentication user

I have (somewhat) large list of jokes in my Firebase Database like in the image below.
I display them in a list in my Android app something like a feed. I also implemented possibility to log in with Firebase Authentication and now I want to add options for logged users to (dis)like jokes and add them to favorites (favorites are supposed to be like private bookmarks). I'm wandering how I could structure my data and I have two proposals:
Add new root node called "userJokes" with child nodes representing user UID from Firebase Authentication. Every UID child node should have copy of every joke from "joke" node with additional booleans representing (dis)like and favorite states.
Another solution is to add every user UID to a joke a user (dis)likes or adds to favorite.
First solution is logical, but how could I count number of likes and dislikes if I structure data this way? And what is the best way to copy every joke from "joke" node to "userJokes" node for every user to be showed in the feed? Second is impractical since while retrieving jokes, I will get info about every user that has (dis)liked or added to favorites and this is not what I need. Which solution is better? Is there any other? Is it OK to add user UID from Firebase Authentication to database in Firebase Database?

I think the first one is more accepted, although it needs some tweak :)
First note: if you create data only to be used as relational (like userJokes), it's better to just add simple value to it without copying entire source data (jokes data), like this:
userJokes: {
randomUserId: {
randomJokeId:true,
anotherRandomJokeId:true
}
awesomeUser: {
randomJokeId:true
}
}
Second note: if you want to implement two functionality (like and favorite), I think you should make it as different data. So it would be userJokeLike and userJokeFavorite (or something like that). And the structure for each of them should be same as I mentioned in first note.
In conclusion:
Every joke data is still in their source path (i.e. inside jokes) and ONLY their id is copied into newly created data path (userJokeLike and userJokeFavorite)
When you want to search for joke that user with id randomUserId likes, you should check for userJokeLike\randomUserId. Then from every joke id you got there, get the real data from inside source jokes path.
When you want to search for joke that is favorited by user with id randomUserId, basically, do the same as above.
When you want to count likes and favorite of each joke, just use something like this:
FirebaseDatabase.getInstance().getReference("userJokeLike")
.orderByChild().equalsTo("randomJokeId")
.addListenerForSingleValueEvent(new ValueEventListener() {
... onDataChange(DataSnapshot dataSnapshot) {
int jokeCount = dataSnapshot.getChildrenCount();
}
});
And there you go, hope this helps.
Note: I haven't check the last code, hope that work :p
EDIT:
Looks like I did misunderstand :D
The solution above is what I think is best for the sake of structure itself. But if we need something simple and fast, it is different for each case/situation. I think that the best solution if you want to get jokes with likes and favorites included (no need to create another request) then your structure should look like this:
jokes: {
randomJokeId: {
// joke data here
likes:{
randomUserId:true,
anotherUserId:true
},
favorites:{
randomUserId:true
}
}
}
It includes likes and favorite when you request jokes data. SO in each data you only need to check if current user's UID is exist inside likes and/or favorite. And the counter will be a lot easier this way.
Happy coding :)

Firebase structuring relationship

I started developing simple app to learn Firebase, I followed cool blog post: https://firebase.googleblog.com/2013/04/denormalizing-your-data-is-normal.html
In my app, I want to store user profiles (its extension of firebase user) and relationship between users (something like friendship)
I came up with this data structure idea:
profile
profile1:
userName:"User 1",
userDescription: "User 1 description"
profile2:
userName:"User 2",
userDescription: "User 2 description"
profile3:
userName:"User 3",
userDescription: "User 3 description"
profileFriends:
profile1:
profile2: true
profile2:
profile1: true
profile3: true
profile3:
profile2: true
Of course instead of profile1 I use pushed keys.
I wonder I its okay for such a use case - I want to display all friends of profile2.
I have to get Database reference to
"profileFriends/profile2"
And then iterating childs gives me keys: profile1 and profile3 which I can then listen using reference
"profile/profile1"
"profile/profile3"
Since Im working in Android, I can wrap all this code and use Observable that emits profiles.
Question: Do I get this right? I have some SQL background and standard request-response api experience, Im just little worried if my user have 100 friends Ill need to make total 101 listeners - is it similar to make 101 requests? Is there any smarter way to solve "join" in non-joinable no-sql database?
I guess another solution is denormalization, but I'm not a big fan of updating many places to change for example profile description

Possible to query against the count of an included key?

I have an application where I need to return the first user found that meets certain criteria, some of that criteria is having a certain number of objects stored.
For example, let's say I want to return the first store I can find that has at-least 3 employees with atleast two children. I know, what an odd-ball example. So I would have a query something like this:
PFUser.query()?
.whereKey("objectId", notEqualTo: PFUser.currentUser()?.objectId!)
.includeKey("stores.employees.children")
// .whereCountForkey("stores.employees", greaterThan: 2)
// .whereCountForKey("stores.employees.children", greaterThan: 1)
.getFirstObject();
Notice the commented out lines, I'm trying to find a way to do soemthing like this in a single query. I'm using parse, which I believe uses MongoDB on the back end, but I don't believe you can execute custom database queries..?
This is a mobile application for both iOS and Android, although the code shown is in SWIFT I have two variations of the project. Examples in either swift, obj-C, Java, or C# will be fine.
Also more than happy with Cloud-code solutions.

There is an example in the documentation
var Team = Parse.Object.extend("Team");
var teamQuery = new Parse.Query(Team);
teamQuery.greaterThan("winPct", 0.5);
var userQuery = new Parse.Query(Parse.User);
userQuery.matchesKeyInQuery("hometown", "city", teamQuery);
userQuery.find({
success: function(results) {
// results has the list of users with a hometown team with a winning record
}
});

How Should I Store Data for My Android App?

I'm quite new to Android programming (very little programming experience). I want to make an app that will track Car maintenance. I would like users to be able to see their data (roughly) according to the following hierarchy:
Year (see total costs, maybe summarize categories)
--Month (month's costs)
----Maintenance Instance
------Details about the instance (what was done for what cost)
I don't have my data design finalized, but you can see the kind of data I'm trying to track. What approach would you suggest? Do I need to use SQLite? If so, would you recommend a hierarchy of tables or just one table that will be shown hierarchically through queries? Like I said, I'm new. I'd appreciate any pointers in the right direction.

In Android, you can use SharedPreferences to store simple data like global preferences (i.e. in your app you could store a currency flag as a preference to display currency as dollars or pounds) but for anything more complicated you should use SQLite. This tutorial is excellent and will get you started - http://www.vogella.com/tutorials/AndroidSQLite/article.html It seems like you could have one table with each row being a maintenance entry with columns for the date, cost and action carried out. You could then query the database by a date range to get the cost for that range or a list of action carried out in that range (e.g. per month or year). Each row would represent a separate maintenance event.

I recommend you use JSON, a very easy to use storage format. A typical JSON message you would store might look like the following:
{
"maintenance_data": [
{
"date": 1091029109,
"maintenance_details": "Drove car around while owner was gone"
},
{
"date": 1021234134,
"maintenance_details": "Ate cookies while on job"
},
{
"date": 1041023234,
"maintenance_details": "Ain't nobody got time for maintenance"
}
],
"car_id": 1234,
"owner_name": "Slick diddy"
}