Develop Reference

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.

How to do search in Room data base?

I have created a room database, and showed my data in a RecyclerView. I know how to add, delete, deleteAll, update data in room data base, but the problem is I don't know how to perform search in room data base?
I have added a search view in the tool bar. But I don't know how to add Queries in NoteDao and the remaining classes, e.g. in my adpter, MainActivity etc.

If you're able to show your Room database in the RecyclerView, this would mean you should know how to use the #Query() tag for fetching your data.
Searching is done using the same #Query tag using the WHERE keyword.
For example, if you wish to find a specific note:
#Query("SELECT * FROM notes WHERE id == :id")
public Note getNote(String id)
However, that's just the simplest way to use WHERE. Frequently, you'll be combining it with other keywords such as IN, LIKE, or BETWEEN.
So for your situation of using a search, you would most likely want to use the LIKE keyword, which can query your database for a field that contains parts of its data that matches the search criteria:
For example:
#Query("SELECT * FROM notes WHERE note_title LIKE :search OR note_message LIKE :search")
public List<Note> getSearchedNote(String search)
This would search your database and return a list of Notes that has either a Note Title or a Note Message that contains the search term you want.

How to join three tables in Firebase Realtime Database?

I have to join three tables in Firebase to fetching the value from it.
consider three table A,B, C.Table A contains attribute as(id1,name).Table B contains attribute as(id2,Bname).C contains attribute as(id1,id2).the relation between table A and table B is M to M.
I want this query to executed in Firebase.
the query is
select A.id1 from A,B,C where A.id1=C.id1 and B.id2=C.id2 and B.Bname="?"
Thank You!.

The Firebase Realtime database is not like an sql database, it has no idea of joins etc. References are fairly cheap, so maybe create 3 references to the 3 locations that contain the data and run these 3 "queries" and tie the results together to get the final view of the data you want. Something like RxJava might help in running the 3 "queries" and chaining the results together.
Try joining the Firebase Slack channel, it has a lot of good posts on how to structure your Firebase data etc.
https://firebase.community/

Android Content Provider SQLite Junction Tables

I'm implementing a Content Provider, which is backed by a fairly complex SQLite DB schema. The database has a few junction tables and I'm unsure whether they should be visible to the user of the Content Provider or not.
The parent tables are exposed via the Contract, each one has its own content URI, etc. Now, when inserting data via ContentResolver#applyBatch() method, I create ContentProviderOperation per each table's content URI. So far everything is clear. But my problem is, how should the junction tables be populated, as they don't have their own content URIs?
To illustrate this, here's an example. I have 2 "parent" tables, Movies and Actors. The relationship between them is many-to-many and therefore I have a junction table called MoviesActors.
To insert at one batch I do the following:
List<ContentProviderOperation> operations = new ArrayList<>;
// movie
operations.add(ContentProviderOperation.newInsert(Contract.Movie.ContentUri).withValue("movie_id", "23asd2kwe0231123sa").build());
// actor
operations.add(ContentProviderOperation.newInsert(Contract.Actor.ContentUri).withValue("actor_id", "89asd02kjlwe081231a").build());
getContentResolver().applyBatch(authority, operations);
The junction table MoviesActors should be inserted with a row containing movie_id and actor_id. How do I take care of the junction table in this situation?
The only thing, which comes to my mind is extend the Contract to have content URI pointing to the junction tables and add another ContentProviderOperation, since otherwise, how do you communicate movie_id and actor_id to ContentProvider#applyBatch()?
I rather not expose the junction table to the user of the ContentProvider, but I might be wrong here... perhaps that's how it should be done on Android?
I've searched this topic for days already and haven't found an answer to that.
Any help would be greatly appreciated.
Bonus question:
Is it necessary to expose every single table via the Contract? For instance, when having child tables in one-to-many relationship. I'm specifically referring to Insert/Update/Delete since I know with Query I can simply do a join, but maybe I'm wrong also here.
Thanks a lot!
NOTE: I'm not interested in 3rd party library solutions.

I think you're tackling the problem from the wrong end. You're trying to design an interface to match your database structure, but the interface should come first.
In the first place, the interface should meet all the requirements of your ContentProvider client. If your ContentProvider client needs access to the junction table you'll have to expose it (in some way, see below), otherwise you don't have to. A good interface hides the actual implementation details, so the ContentProvider client doesn't need to care about whether the ContentProvider is backed by an SQLite database, by a bunch of in-memory maps or even a web-service.
Also, you should not think of a ContentProvider just as an interface to a database and the Contract as the database schema. A ContentProvider is much more versatile and powerful than that. The major difference is that ContentProviders are addressed by URIs whereas in SQL you just have table names. In contrast to a table name, a URI has a structure. URIs have a path that identifies the object (or directory of objects) that you want to operate on. Also you can add query parameters to a URI to modify the behavior of an operation. In this respect a ContentProvider can be designed much like a RESTful service.
See below for a concrete (but incomplete) example of a Contract of a simple movie database. This is basically how one would design a RESTful web-service, except for one thing: Just like in your code, movie-id and actor-id are provided by the caller. A real RESTful service would create and assign these automatically and return them to the caller. A ContentProvider can only return long IDs when inserting new objects.
Insert a new movie
insert on /movies/
Values: {"movie_id": <movie-id>, "title": <movie-title>, "year": ...}
Insert a new actor
insert on /actors/
Values: {"actor_id": <actor-id>, "name": <actor-name>, "gender": ...}
Add an existing actor to a movie
insert on /movies/movie-id/actors/
Values: {"actor_id": <actor-id>}
Add an existing movie to an actor:
insert on /actors/actor-id/movies/
Values: {"movie_id": <movie-id>}
Optional: add a new actor directly to a movie:
insert on /movies/movie-id/actors/
Values: {"actor_id": <actor-id>, "name": <actor-name>, "gender": ... }
If no actor with the given id exists, this operation will create the new actor and link it to the movie in a single step. If an actor with this ID already exists an exception would be thrown.
The same could be done the other way round, adding a new movie to an actor.
Delete an actor from a movie
delete on /movies/movie-id/actors/actor-id
or
delete on /actors/actors-id/movies/movie-id
Get all movies
query on /movies/
Get a specific movie
query on /movies/movie-id
Get all actors playing in a specific movie
query on /movies/movie-id/actors/
Get all movies a specific actor has played in
query on /actors/actor-id/movies/
The optional query selection statement can be used to filter the result. To get movies from the last 10 years a specific actor has played in, you would add the selection movies_year>=2005 to the last query.
By using a contract like this you wouldn't expose the junction table, instead you provide a REST-like interface to your database.
The job of the ContentProvider is to map these operations onto the database or any other back-end.

Android - Complex Databases and Content Providers

I am designing an Android app with a fairly complex (normalized) database structure. My question is, what is the best design pattern for working with my data?
The database has a table Books, a table Authors, a table Collections (of books), and a junction table Books_Collections that relates the two based on an integer ID. I say that it is fairly complex because I want to display a list of books & authors in a specific a collection in a ListView in format "Book Title (Author Name)", so I need to be able to perform the following query (I will abbreviate the table names A, B, C, and B_C):
SELECT B.title, A.name
FROM B
JOIN A ON B.author_id = A.id
JOIN B_C ON B_C.book_id = B.id
WHERE B_C.collection_id = [variable]
I have seen some people advising developers to wrap their Sqlite databases in a Content Provider to easily take advantage of the Loaders introduced in Honeycomb. But is this really feasible for such complex queries? (If so, how?) or would it be better to just use DAO since I am not sharing my data with other apps? And if using DAO, should a custom AsyncTaskLoader be created to connect the data to a ListView?
EDIT
One more important detail: I want to change the background color of the list items based on a boolean variable in Books marking whether or not the book has been read, and a context menu will allow users to delete books from the list, so the list needs to adjust instantly to data changes.

Yes, is doable and easily done
I'm no expert but im followin the pattern used by google on his Google I/O app
https://code.google.com/p/iosched/
Check the provider package where u have the 3 classes needed for using the pattern
ScheduleDatabase.java
(definition of sqlite3 database)
ScheduleProvider.java
(Content provider atttached to this database)
ScheduleContract.java
(Contract defined to expose the provider)
Personally, in the case of join
in the Database File i define
interface Tables {
String TABLE1= "table1";
String TABLE2= "table2";
// JOINS
String TABLE1_JOIN_TABLE2 = "table1"
+ "LEFT OUTER JOIN table2 ON table1.table_id= table2.tableid";
}
and then in your provider map a provider entity to this join.

I ended up using database helpers/adapters and creating a custom AsyncTaskLoader. I wasn't able to find a ton of examples of custom ones, and there's not great documentation on it, but it's not too hard to figure out. Works like a charm.