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How can I access the child of a unique key in Firebase?

I am trying to access the child value of a unique key value (that had been "pushed") within Firebase. Currently, my database looks like this: I want to access the value of "emailOfUser"
I am very new to Firebase so I am not familiar with the functions. Currently, this is my method of obtaining other values for a different section of the database:
Thank you so much for any feedback!
I've tried different methods to accessing this data within the Firebase, but I cannot get it to work/the methods I were using were outdated. I also tried to "update" the Firebase instead of "pushing" the values to prevent a unique key from generating, but it simply overwrote my current data rather than appending something new.

If you want to load all the users who voted and print their emails, you can do that with:
get(child(dbref, 'usersWhoVoted')).then((snapshot) => {
snapshot.forEach((childSnapshot) => {
console.log(childSnapshot.key, childSnapshot.val().emailOfUser);
});
})
Note that your current structure allows a user to vote multiple times. If you want to only allow them to vote once, use some identifier of the user as the key in your database structure:
userVotes: {
"uniqueIdOfUser1": "valueTheyVotedOn",
"uniqueIdOfUser1": "valueTheyVotedOn",
...
}
Now each user can by definition only vote once, If they vote again (assuming your security rules allow that), their new vote will simply replace the existing vote.

How to only read changes in firebase realtime database

I have a database collection with readings, each new reading needs to be checked if it's out of the ordinary, if it is, there needs to be an alert sent.
So i'm using db.ref('collection').on('child_added', (child => { check(child); });
The problem with the .on function is that when the listener is added, all previous data is also read.
So how do i read a collection that only reads the changes in the database, also when the listener is first added? Or if that doesn't work, how do I differentiate the already added data with the new data?

The Firebase database synchronizes the state of whatever query or reference you attach your listener to. There is no option to only get new nodes built into the API.
If you want only new nodes, you will have to:
Ensure each node has an associated timestamp or order. If you're using Firebase's built-in push() keys, those might already serve that function.
Know what "new" means to the client, for example by either keeping the last timestamp or push key that it saw.
And then use a query to only request nodes after the stores timestamp/key.
So for example, if you only want to read nodes that are created after the moment you attach the listener, you could do something like this:
let now = db.ref('collection').push().key; // determine current key
db.ref('collection').orderByKey().startAt(now).on('child_added', ...)

Address Firebase object by child value, and then modify that object?

I'm facing a little difficulty finding information about how to modify objects in Firebase Realtime Database. I'm adding items into my database in real-time, so I won't know ahead of time what the object key is. As you know, the database structure looks like this:
Say I want to address a child of "testing", whose category is "social", what I do is this:
firebase.database().ref("testing").orderByChild("category").equalTo("social")
But how exactly can I then address this child so as to update this entire child (including all the fields - "category", "date", etc.) or even delete it? Thanks so much for any help!

To update (or delete) a node in Firebase you must know the complete path to that node. If you don't know the complete path, you can use a query to determine the node(s) matching a certain condition.
So in your case you'll need to execute the query, loop over the results, and update each child node in turn. In code:
let query = firebase.database().ref("testing").orderByChild("category").equalTo("Social");
query.once("value").then((results) => {
results.forEach((snapshot) => {
snapshot.ref.update({ propertyToUpdate: "new value" });
});
});
If you want to delete the matching node(s), the innermost line would be: snapshot.ref.remove().

Firestore data modeling and angularFire

I have data model like this
Players-->root collection
Sports--->root collection
Players_Sports---root collection
I want get all the sports(Multiple sport) details or document player belongs.
For this I am using angularFireStore5
First, I am getting
Player details like this
this.db.collection('players').doc(id).get().toPromise()
Second, I am getting Player(user) linked PlayerSport
db.collection<any>('PlayerSports',ref => ref.where('playerId', '==', id) ).get().toPromise()
Third, I am trying to get Sports details based on ID'S,
db.collection<any>('sportsType', ref => ref.where('sportsType' 'in' [sportsIDs])).get().toPromise()
where SportIDs is arrary of ID that are linked in player_sports
First and Second steps works fine, but third steps is not giving any data or response
Could you please let me know where is the problem,
is it in Data model or code? my guess is that data model is not correct. Please guide me on this.

I would suggest getting the data from firebase and storing it inside a list so the app can access it later.
void getDataF(){
databaseReference
.collection("TableName")
.getDocuments()
.then((QuerySnapshot snapshot) {
snapshot.documents.forEach((f) {
iDFacList.add(f.documentID);
dbFacList.add(f.data["FieldName"]);
});
});
}

There is no sportsType field in the sportsType document as far as I can see.
If you're trying to find documents based on their SportsId field, you'll want ref.where('SportsId'....
Update
It seems that you're trying to find a document by its ID, which you can do with:
ref.doc(sportsIDs)
If you want to get multiple documents, or get a single document as a collection, you can use:
ref.where(firebase.firestore.FieldPath.documentId() 'in' [sportsIDs])

Breeze JS: Is there a way to query entities from data.results?

I have a Breeze web api controller, with methods that accept parameters and do some work, filtering, sorting, etc, on the server.
On the querySucceeded, I'd like to do further querying to data.results. Is there a way to accomplish this? I got this working by exporting/importing data.results to a local manager, and do the projection from there. The projection is needed in order to use the observable collection in a vendor grid control.
var query = datacontext.EntityQuery.from("GetActiveCustomers")
.withParameters({ organizationID: "1" })
.toType("Customer")
.expand("Organization")
.orderBy('name');
var queryProjection = query
.select("customerID, organizationID, name, isActive, organization.name");
return manager.executeQuery(query)
.then(querySucceeded)
.fail(queryFailed);
function querySucceeded(data) {
var exportData = manager.exportEntities(data.results);
var localManager = breeze.EntityManager.importEntities(exportData);
var resultProjection = localManager.executeQueryLocally(queryProjection);
//This is the way I came up with to query data.results (exporting/importing the result entities to a local manager)
//Is there a better way to do this? Querying directly data.results. Example: data.results.where(...).select("customerID, organizationID...)
if (collectionObservable) {
collectionObservable(resultProjection);
}
log('Retrieved Data from remote data source',
data, true);
}

You've taken an interesting approach. Normally a projection returns uncacheable objects, not entities. But you casted this to Customer (with the toType clause) which means you've created PARTIAL Customer entities with missing data.
I must hope you know what you are doing and have no intention of saving changes to these customer entities while they remain partial else calamity may ensue.
Note that when you imported the selected Customers to the "localManager" you did not bring along their related Organization entities. That means an expression such as resultProjection[0].organization will return null. That doesn't seem correct.
I understand that you want to hold on to a subset of the Customer partial entities and that there is no local query that could select that subset from cache because the selection criteria are only fully known on the server.
I think I would handle this need differently.
First, I would bury all of this logic inside the DataContext itself; the purpose of a DataContext is to encapsulate the details of data access so that callers (such as ViewModels) don't have to know internals. The DataContext is an example of the UnitOfWork (UoW) pattern, an abstraction that helps isolate the data access/manipulation concerns from ViewModel concerns.
Then I would store it either in a named array of the DataContext (DC) or of the ViewModel (VM), depending upon whether this subset was of narrow or broad interest in the application.
If only the VM instance cares about this subset, then the DC should return the data.results and let the VM hold them.
I do not understand why you are re-querying a local EntityManager for this set nor for why your local query is ALSO appling a projection ... which would return non-entity data objects to the caller. What is wrong with returning the (partial) Customer entities.
It seems you intend to further filter the subset on the client. Hey ... it's a JavaScript array. You can call stuffArray.filter(filterFunction).
Sure that doesn't give you the Breeze LINQ-like query syntax ... but do you really need that? Why do you need ".select" over that set?
If that REALLY is your need, then I guess I understand why you're dumping the results into a separate EntityManager for local use. In that case, I believe you'll need more code in your query callback method to import the related Organization entities into that local EM so that someCustomer.organization returns a value. The ever-increasing trickiness of this approach makes me uncomfortable but it is your application.
If you continue down this road, I strongly encourage you to encapsulate it either in the DC or in some kind of service class. I wouldn't want my VMs to know about any of these shenanigans.
Best of luck.
Update 3 Oct 2013: Local cache query filtering on unmapped property
After sleeping on it, I have another idea for you that eliminates your need for a second EM in this use case.
You can add an unmapped property to the client-side Customer entity and set that property with a subset marker after querying the "GetActiveCustomers" endpoint on the server; you'd set the marker in the query callback.
Then you can compose a local query that filters on the marker value to ensure you only consider Customer objects from that subset.
Reference the marker value only in local queries. I don't know if a remote query filtering on the marker value will fail or simply ignore that criterion.
You won't need a separate local EntityManager; the Customer entities in your main manager carry the evidence of the server-side filtering. Of course the server will never have to deal with your unmapped property value.
Yes, a breeze local query can target unmapped properties as well as mapped properties.
Here's a small demonstration. Register a custom constructor like this:
function Customer() { /* Custom constructor ... which you register with the metadataStore*/
// Add unmapped 'subset' property to be queried locally.
this.subset = Math.ceil(Math.random() * 3); // simulate values {1..3}
}
Later you query it locally. Here are examples of queries that do and do not reference that property:
// All customers in cache
var x = breeze.EntityQuery.from("Customers").using(manager).executeLocally();
// All customers in cache whose unmapped 'subset' property === 1.
var y = breeze.EntityQuery.from("Customers")
.where("subset", 'eq', 1) // more criteria; knock yourself out
.using(manager).executeLocally();
I trust you'll know how to set the subset property appropriately in your callback to our "GetActiveCustomers" query.
HTH

Once you queried for some data breeze stores those entities on the local memory.
All you have to do is query locally when you need to filter the data some more.
You do this by specifying for the manager to query locally :
manager.executeQueryLocally(query);
Because querying from the database is done asynchronously you have to make sure that you retrieve from the local memory only if there is something there. Follow the promises.