Develop Reference

Retrieve multiple data in recycler view using FirebaseRecyclerOptions

I am using FirebaseRecyclerOptions in calling the database, however, I cannot get all of the data in the database. Here is the structure of the database: database structure the yellow underline is the user id (UID) and below is another node that contains the data that I want to retrieve in the RecyclerView.
Here is a snippet of the code
FirebaseRecyclerOptions<RegisterParking> options =
new FirebaseRecyclerOptions.Builder<RegisterParking>()
.setQuery(FirebaseDatabase.getInstance().getReference().child("RegParkingArea"), RegisterParking.class)
.build();
voPListAdapter = new VoPListAdapter(options);
recyclerView.setAdapter(voPListAdapter);

When you're passing the following two arguments to the setQuery() method:
.setQuery(FirebaseDatabase.getInstance().getReference().child("RegParkingArea"), RegisterParking.class)
It means that the adapter expects to render on the screen RegisterParking objects. If you take a closer look at your database schema, under the RegParkingArea node, you can find UIDs and not RegisterParking objects. The objects that you want to display in the RecycerView exist under each UID. So when reading the data from the database, the Firebase-UI library tries to map each child under the above reference into an object of type RegisterParking, which is actually not possible since the UIDs are strings.
So if you're allowed to change the database schema, then you should either denormalize the data, basically copying the data under another node, or change the actual data into:
db-root
|
--- RegParkingArea
|
--- $pushedId
|
--- uid: "4u9h...XrP2"
|
--- //other fields.
What I have basically done, I have removed a level from the database tree. If you'll use this schema, then you can leave the code as it is and everything will work perfectly fine.
One more thing to note is that if you need to get all parking of a particular user, then you can use the following query:
DatabaseReference db = FirebaseDatabase.getInstance().getReference();
Query queryByUid = db.child("RegParkingArea").orderByChild("uid").equalTo(uid);

Save button deletes previous data in firebase

I am new to android studio and firebase. I am trying to save a list of people to firebase like this. Idea is that the logged in user should be able to save information about some people.
String userId = user.getCurrentUser().getUid();
databaseReference.child("users").child(userId).child("savedPersons").child("name").setValue(nameTxt);
databaseReference.child("users").child(userId).child("savedPersons").child("surname").setValue(surnameTxt);
databaseReference.child("users").child(userId).child("savedPersons").child("gender").setValue(genderTxt);
databaseReference.child("users").child(userId).child("savedPersons").child("ageTxt").setValue(ageTxt);
It does not surprise me that it deletes the previous saved person when i save another one but i don't know how to save all of them. I have this in my firebase but i need multiple saved users. How do i do it ?
Firebase screenshot

If you want to save multiple people in a list in the database, you'll want to call push:
String userId = user.getCurrentUser().getUid();
DatabaseReference newRef = databaseReference.child("users").child(userId).child("savedPersons").push(); // 👈
newRef.child("name").setValue(nameTxt);
newRef.child("surname").setValue(surnameTxt);
newRef.child("gender").setValue(genderTxt);
newRef.child("ageTxt").setValue(ageTxt);
This will create a new child node under savedPersons each time you call push(). To learn more on this, see the Firebase documentation on appending data to a list.
Note that calling setValue for each property is wasteful, and may lead to unexpected behavior down the line. I recommend putting all values in a map, and then adding them all with one call to setValue:
Map<String, Object> values = new Map<>();
values.put("name", nameTxt);
values.put("surname", surnameTxt);
values.put("gender", genderTxt);
values.put("ageTxt", ageTxt);
newRef.setValue(values);

How to show items on RecyclerView Android based on the values of a child

I need to retrieve some data on Firebase realtime database based on the values of a child.
For example, here I need to show only the child(notification) where accepted==true.
I pass my values on RecyclerView with a reference without creating a list.

To get all notifications "WHERE" accepted property holds the value of true, you need to use a query. In code, looks like this:
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference notificationsRef = rootRef.child("Notifications");
Query query = notificationsRef.orderByChild("accepted").equalTo(true);
query.addListenerForSingleValueEvent(/* ... */);

Have you tried this?
https://firebase.google.com/docs/database/android/lists-of-data#filtering_data
You can use equalTo() method to filter data.

How make complex query with FirestoreRecyclerAdapter?

I'm developing an Android app and I need to make a complex query on my Firestore database and create a FirestoreRecyclerAdapter. To create the adpater I need a FirestoreRecyclerOptions object that take in input the whole query. Reading the documentation, I can't use in my query the methods whereGreaterThan, whereLessThan, oderBy, etc, on different parameters. For example, how can I get users from db who have age greater than/less than AND who have weight greater than/less than?
For example the document's structure in my firestore database is:
Users --->UserID--->userName(String)
--->userAge(number)
--->userHeight(number)
--->userWeight(number)
FirebaseFirestore db;
db = FirebaseFirestore.getInstance();
RecyclerView recyclerView;
recyclerView = (RecyclerView)findViewById(R.id.recyclerViewID);
.
.
.
Query query = db.collection("Users").//the complex query that i need
FirestoreRecyclerOptions<User> options = new FirestoreRecyclerOptions.Builder<User>()
.setQuery(query, User.class)
.build();
adapter = new UsersAdapter(options, this);//get in input options and the context
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setAdapter(adapter);
Edit: A possible solution in my last comment to answer 1

There are some query limitations when it comes to Firestore:
Query limitations
Cloud Firestore does not support the following types of queries:
Queries with range filters on different fields, as described in the previous section.
So you cannot query your database on range filters using whereGreaterThan() and whereLessThan() methods on different properties and pass it to an adapter.
Edit:
A possible solution for this issue would be to filter your records client side twice. First query the database using the first property and second using the second property. Unfortunately you cannot achieve this in a single go.
Edit2:
The solution would be to query the database using the first query, get the corresponding elements, query the database again using the second query and get the corresponding elements and then merge the results client side. Now the elements from the database were filtered twice. Pass a list of that elements to an adapter and that's it. Note, when using this solution you cannot use the Firebase-UI library anymore but this how you can get your items filtered twice.

How to arrange data from Firestore in ListView by biggest int values

I had a Firestore collection named "Users" and Documents of user names , and an integer field of every username , now i wanna compare all those fields and put the biggest one on the top of the list and so on till 10 items , thanks .

Assuming that you have the users of your app added as documents within the Users collection and the name of the property is score, to achieve this, please use the following code:
FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
Query query = rootRef.collection("Users")
.orderBy("score", Query.Direction.DESCENDING)
.limit(10);