Develop Reference

Firebase database - insert record in sequence - Android [duplicate]

I have a node for users with key 1 to 1400 but it's not in order.
Hot to view this in order in the console. I had a similar experience with another node. But it later got ordered automatically

Unfortunately, you cannot change the order of the nodes in the Firebase Database Console. By default, all the nodes are ordered by key. One thing to remember is that Firebase keys are Strings. And when strings are order, are ordered lexicographically.
So for numbers, this is the normal order:
1308
1309
1310
1311
But for strings, this is the normal order:
"1308"
"1309"
"131"
"1310"
There is no operator in Firebase and as far as i know nor in most other databases that allow you to change this behavior. Instead, you will have to modify the data to get the behavior you want. So, store values that are in the order you need them when sorted lexicographically. For numbers you can accomplish that by padding them with zeroes:
"0131" //zero added before
"0132" //zero added before
......
"1308"
"1309"
"1310"
"1311"

Firebase Realtime database Array query issue [duplicate]

This question already has an answer here:
Firebase query if child of child contains a value
(1 answer)
Closed 3 years ago.
I need a firebase query to filter the list based on the value of an array.
if any of the index of GID(Array) contains the given key. e.g my key is YsGMyfLSGRNHxDWQmhpuPRqtxlq1 and one node's GID have that on 0th index and other have that on 1st index. So these two lists need to be returned.
Currently, I can only get the one at 0th index using the code
//userID = YsGMyfLSGRNHxDWQmhpuPRqtxlq1
firebaseDatabase.child("Groups").queryOrdered(byChild: "GID/0").queryEqual(toValue:userID)
When I try to combine the query I am getting errors.

I don't know about your database structure, But I can explain that There is a limitation in Firebase Realtime database that you can only order by 1 child.
So now if we require to order by 2 Childs we can combine 2 nodes and make it 1 node and can apply order by query on it. For example
If we have username & email fields we can make a new field username_email and can apply order by on it.
Like
user: {
username: "john",
email: "john#g.com"
username_email = "john_john#g.com"
}
Now we can write
firebaseDatabase.child("user").queryOrdered(byChild: "username_email").queryEqual(toValue: "john_john#g.com");

There is no way you can filter your groups based on a value that exist within an array. If you want to query your database to get all groups a particular user is apart of, then you should consider augmenting your data structure to allow a reverse lookup. This means that you should add under each user object the groups in which that user is present.
This means that you'll need to duplicate some data, but this is not a problem when it comes to Firebase. This is a quite common practice, which is named denormalization and for that, I recommend you see this video, Denormalization is normal with the Firebase Database.
When you are duplicating data, there is one thing that need to keep in mind. In the same way you are adding data, you need to maintain it. With other words, if you want to update/detele an item, you need to do it in every place that it exists.
However, what you need is actually allowed in Cloud Firestore. Its array-contains operator allow you to filter documents that have a certain value in an array. For more on this topic, please see the following post:
Better Arrays in Cloud Firestore.

Firebase matching substring

I have the JSON structure above
I want to get the list of vehicles where their ids ends with "123"
I had tried to use Query.endAt() method but i'm not sure if i'm using it right or it shouldn't give the required output
Query vehiclesRef;
vehiclesRef = db.getReference("vehicles").orderByKey().endAt("\uf8ff123");

Firebase Database queries can only perform prefix matches, so strings starting with a specific string or starting with a range of string. It is not possible to query for strings ending with a specific value, nor those ending with a range of values.
If you really want to do this within Firebase, consider also storing the reversed strings in the database:
"321_1_0"
"321_3_0"
"654_52_0"
Then you can query for strings starting with 321 with
vehiclesQuery = db.getReference("vehicles").orderByKey().startAt("321").endAt("321\uf8ff");

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.

ORDER BY Date - Realm (Android)

I've a column/field called last_message_time of type Date in my Table A.
Suppose querying Table A returns x results.
How do i sort these results based on dates inside last_message_time column.
Example, in SQLite we have ORDER BY date(dateColumn)

RealmResults<A> sorted = realm.where(A.class)
.findAllSorted("last_message_time", Sort.ASCENDING);
EDIT: since Realm 4.3.0, the following is preferred:
RealmResults<A> sorted = realm.where(A.class)
.sort("last_message_time", Sort.ASCENDING)
.findAll();

Use just "sort"!
"findAllSorted" is deprecated!
io.realm.RealmQuery.findAllSorted(String)
Since 4.3.0, now use RealmQuery.sort(String) then RealmQuery.findAll() Finds all objects that fulfill the query conditions and sorted by specific field name in ascending order.
Sorting is currently limited to character sets in 'Latin Basic', 'Latin Supplement', 'Latin Extended A', 'Latin Extended B' (UTF-8 range 0-591). For other character sets, sorting will have no effect.
More details in: LINK