Develop Reference

Filter and get all sub-collections by using where clause in Firestore

In firestore I have some sub-collections inside each collection just like below:
Collection1 -> id -> Sub-collection -> scId -> Data
So here I want to get all data from sub-collections by the help of the where clause. I tried one query like the below which is failing.
db.collection("Collection1").whereEqualTo("eDate", selectedDate)
.get().addOnSuccessListener(queryDocumentSnapshots -> {
List<Exp> mData = queryDocumentSnapshots.toObjects(Exp.class);
Here eDate is coming under the Data which is inside each sub-collection.
So is there any way to do that or any suggestions on how to do that or about the mistakes which I did here?

Firestore queries are shallow and do not extend above or below the named collection being queried. The query you show here will only return documents immediately within "Collection1". It will not consider any documents in nested subcollections.
If you want documents in a subcollection, you will have to build a CollectionReference to that subcollection, and query it individually. You can't query all subcollection nested under a document at the same time. You can't query across differently-named subcollections at the same time.
If this behavior does not meet the needs of your app, you should consider restructuring your data to do so, or building your app to perform as many queries as needed to get all the documents from among all the collections where they live.

Firebase Firestore OR query (Android Studio)

I've been trying to find a query for almost 2 days now
I want to search id (current user id) from the document 4 fields (customer1,customer2,customer3,customer4)
Here is the firestore document picture
tried this query
final Query userQuery = collectionReference
.whereEqualTo("customer1",firebaseAuth.getInstance().getCurrentUser().getUid())
.whereEqualTo("customer2",firebaseAuth.getInstance().getCurrentUser().getUid())
.whereEqualTo("customer3",firebaseAuth.getInstance().getCurrentUser().getUid())
.whereEqualTo("customer4",firebaseAuth.getInstance().getCurrentUser().getUid());
but this only shows up if the current ID is present in all 4. Is there any easier way to do this.

You can do that by using a field that is an array containing the uids you want to test, and then applying array-contains on it. In your case:
In your case:
customer: [customer1, customer2, customer3, customer4]
collectionReference
.where("customer ", "array-contains", firebaseAuth.getInstance().getCurrentUser().getUid())

Firestore does not support logical OR queries among mulitple fields. So, what you're trying to do is not possible with a single query using the database structure you have now. You would have to perform multiple queries and merge the results in the client.
If you want to be able to use a single query, you will have to change your database. One option is to put all the customers into a single array field and use an array-contains query to find a customer in that field.

Android's Firestore to join 2 collections into a recycler view

I have a users collection with uId, name, photo
I have a visits collection with uId, userId, location
I have a recyclerview in which I want to show the location with the user name and photo
Can I use the reference field type? If so, how will Firestore know to link visits.userId == users.uId ?
Maybe I first need to query all the visits and then query the relevant user but 2 things:
It means querying a lot of times.
I didn't understand how to collect the joined collection into the adapter, which is based on one query?
Please advice
Thanks
current code
visitsList = db.collection("visitsList");
Query query = visitsList.whereEqualTo("userId",prefs.getString("id","")).orderBy("visitDate", Query.Direction.ASCENDING);
FirestoreRecyclerOptions<AVisit> options = new FirestoreRecyclerOptions.Builder<AVisit>().setQuery(query, AVisit.class).build();
adapter = new VisitsListAdapter(options, VisitsListActivity.this);
RecyclerView rv = findViewById(R.id.rvVisitsList);
rv.setHasFixedSize(true);
rv.setLayoutManager(new LinearLayoutManager(this));
rv.setAdapter(adapter);
The code is a simple query from the collection, not sure how to get the name and photo from the userId field in that collection.

Can I use the reference field type?
Yes, you can use a reference field.
If so, how will Firestore know to link visits.userId == users.uId ?
Firestore results always comes from a single collection (at the moment). It does not automatically join the document from the users collection when you're reading from the visits collection. You will have to do this yourself.
That indeed means you'll be executing multiple reads, but it's often not nearly as slow as you may think. See Google Firestore - how to get document by multiple ids in one round trip?
Update: To show data from the user profile in a list of visits, there are two main options:
load the additional user document in populateView or with a custom parseSnapshot implementation.
duplicate the relevant user data in the visits collection (which is quite normal in NoSQL databases). Also see Alex' answer here: indexed query with FirestoreRecyclerAdapter.

Firebase query implementation

If we query data from Firebase on a key for a child node, does it downloads the whole child node and filter data in the application or it downloads the query specific data? i.e. the filtered data
String myUserId = getUid();
Query myTopPostsQuery = databaseReference.child("user-posts").child(myUserId).orderByChild("starCount");
myTopPostsQuery.addChildEventListener(new ChildEventListener() {});
myTopPostsQuery will sort data according to starCount and I will receive data in the addChildEventListener() I want to know that whether this data is being filtered inside my app after receiving or my app downloads only the filtered data from the Firebase.

If you use a filter in your query you will download the query specific data without any operation executed on the client side.
Keep in mind that:
You can only use one order-by method at a time. Calling an order-by method multiple times in the same query throws an error.
You can combine multiple limit or range functions. For example, you can combine the startAt() and endAt() methods to limit the results to a specified range of values.
For any other info take a read here

You get the data already filtered. When you use a query, let say, limitToLast(10) you get only those 10 elements. This is happening also in your case with orderByChild("starCount"). In your SnanpShot will find only those filtered elements. Please visit official doc for more details.
Hope it helps.

Based on the code you pasted here, your query will just retrieve all the posts for the database path user-posts/<id> with an ordered manner which means that there is not filter. But still you will get back all the available posts under the path you are querying. It can be a "semi-filter" as it will find only the posts which include starCount field
The best thing is to filter during your query in order to retrieve back exactly what is needed and not everything as you are doing right now, imagine that this list of posts can be really big so you will have a big issue with performance later.
Read the following section here about sorting & filtering.
https://firebase.google.com/docs/database/admin/retrieve-data#orderbychild
Apart from that consider to add an index in userId field for speeding up your query.

Doing queries on firebase

I use firebase in my android app, but i'm facing problems. For example. I have this entrie
persons
-kskhdhksdhh
name:"Vladimir"
age:"25"
sexe:"M"
country:"Russia"
-jhgsdhghsgs
name:"Victoria"
age:"20"
sexe:"F"
country:"England"
-zejhbshevzh
name:"John"
age:"44"
sexe:"M"
country:"England"
I want to select males living in England. How do I rearrange my data to get there knowing we can not do two orderby

First - take a look at this answer , for similar question:
Query based on multiple where clauses in firebase
Anyway, you can add country level to your data structure, so your data can look something like:
persons
-Russia
-kskhdhksdhh
name:"Vladimir"
age:"25"
sexe:"M"
-England
-jhgsdhghsgs
name:"Victoria"
age:"20"
sexe:"F"
-zejhbshevzh
name:"John"
age:"44"
sexe:"M"
Then you can easily query for males living in England.
Another option that described in the link on the top , is to add a key that combine both country & gender, and then query this property.