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Android Room - Create custom index

I migrate my database from DBFlow to Room finally.
However, some queries I made for my old database don't really match what I know about Room. So in my Entity I have / had these calls:
"CREATE INDEX IF NOT EXISTS `idDeletedStartEndLocalIndex` ON `QUANTITY`(START_LOCAL, END_LOCAL DESC)"
I implemented that in Room as
Index("START_LOCAL", "END_LOCAL")
but how can I add the Descending at the index? Should I just write "END_LOCAL DESC"? Would that work as expected?
Same for this one
"CREATE UNIQUE INDEX IF NOT EXISTS `serverQtyId` ON QUANTITY(SERVER_QUANTITY_ID) WHERE SERVER_QUANTITY_ID > 0"
How can I add the WHERE SERVER_QUANTITY_ID > 0 clause to the Index annotation of room? Is that even possible?

Okay, looks like there are not many ways around this. So I did the queries manually.
For those who have the same problem, my code looks like this:
runBlocking(Dispatchers.IO){
with (getInstance().openHelper.writableDatabase) {
execSQL("CREATE INDEX IF NOT EXISTS `someIndexName` ON `QUANTITY`(...)")
...
}
}
So what I do is basically get the Database Instance and their openHelper. From there you can get the writable Database. It then supports executing SQL queries directly.
All this is run in the IO coroutine scope but that's just for convenience.

Room DB Upgrade need to write full insert query

Hi I am developing an android app and I am using room DB and I have created one table and now I want to add another table I need to write a migration for that and in-migration I need to write full SQL queries for the new table assume I have more than 20 fields how complex will be the query.
In SQLite, we need to write such complex queries which sometimes becomes complicated to write and find errors So, room DB came to resue but now we need to do the same in the room DB migration. How does it useful then?
If we use SQLite, then if we already added one table and now We want to add another table then we can just uninstall the application and new tables will be generated, but in-room DB this is not a case I tried the same thing but it is still showing me that you need to write a migration for the new table. So, in this case, while developing there will a lot of migration scripts that will be hard to maintain at some point in the future.
How does it useful then I have to write a multiple create queries while developing the app this is a very basic flow in any application.
Once we go to prodution then it makes sense to write migration for every table but not in the developing mode.
How does room DB make developr job eaiser?

I have more than 20 fields how complex will be the query.
It can be very simple as an Entity defines an Object e.g. your 20 columns and to get the 20 columns can be as simple as
#Query(SELECT * FROM thetable)
List<Thetable> getAll();
The above being in an Interface that is annotated with #Dao and all you do in the code is retrieve an instance from the built room database and then use the getAll method which returns a List of Thetable objects. Each with all the member variables populated from the database.
e.g. you could have :-
mMyTheTableDaoObject = mMyBuiltRoomDatabase.getAll();
List<TheTable> myTheTableList = mMyTheTableDaoObject.getAll();
for(TheTable t: myTheTableList) {
int current???? = t.get????();
}
While using standard/non-room then you would have to do something along the lines of :-
SQLitedatabase db = whatever_you_need_to_do_to_get_an_SQLiteDatabase_instance;
Cursor c = db.query("theTable",null,null,null,null,null,null);
ArrayList<TheTable> myTheTableList = new ArrayList();
while(c.moveToNext()) {
currentTheTable = new TheTable();
current.TheTable.setId = c.getLong(c.getColumnIndex("id");
current.TheTable.setNextColumn1 = c.getString("next_column1");
current.TheTable.setNextColumn2 = c.getString("next_column2");
........ another 17 similar lines of code
currentTheTable.setNextColumn20 = c.getString("next_column20");
myTheTableList.add(currentTheTable);
}
for(TheTable t: myTheTableList) {
int current???? = t.get????();
}
If we use SQLite, then if we already added one table and now We want to add another table then we can just uninstall the application and new tables will be generated, but in-room DB this is not a case I tried the same thing but it is still showing me that you need to write a migration for the new table.
Once we go to production then it makes sense to write migration for every table but not in the developing mode.
Rather then migrating simply delete the database (delete the App's data or uninstall the App, the database is stored in the default location (data/data/package_name/databases)) and rerun without changing the version. The database will be created as per the new schema. Perhaps utilising temporary code to load data accordingly.
How does room DB make developr job eaiser?
In Short ROOM generates what is termed as the boilerplate code from relatively simple code e.g the #Query above writes the underlying code to extract the data and build the objects (e.g. the code as above).

Please check the official document:
https://developer.android.com/training/data-storage/room/migrating-db-versions
Actually Room using SQLITE behind the scene. It provide you plethora of other facilities. In case of Migration you have to write full code to create table.

Harsh your question is valid in some way but as you know android is maintaining SQLite database and libraries like room, greendao or even native SQLiteOpenHelper
is handling the transaction with sqllite behind the scene for the developers.
In all the earlier libraries too you have to maintain versions of your database and which fields or tables have been added to your database and write migrations for the version upgrades of the database.
The beauty of room comes in play in how easy they have made the CRUD operations on the SQLite database and getting data wrapped in LiveData or Observable, not that you don't need to write migrations.

What is the best way to generate Android Room Migrations?

Note that I come from a .NET background where I'm familiar with Entity Framework.
So in EF migrations, the actual migrations are generated for you. Basically, you specify in your POCO objects the changes you want. When you add a new migration, it does a diff between the database and the POCO objects and generates a migration script. You barely have to even look at it.
So using Android Room, I'm finding I have to handcraft these migrations myself. And there are rules that mean the system crashes unless the migration you handcraft puts the schema in exactly the state Room thinks it should be based on the POJO objects.
For me, this seems like a tedious and risky task that could be automated (like EF migrations already does). So my question is, is there a tool or something that'll do these migrations automatically? If not, what are some guidelines on how to do these migrations safely or efficently?

I'm not sure if a tool like this exists specifically for Room as of today, but I hardly doubt it, since Room's stable version was released just a few months ago.
One small thing you can get generated is SQL statements for tables creation.
Just add:
javaCompileOptions {
annotationProcessorOptions {
arguments = ["room.schemaLocation":
"$projectDir/schemas".toString()]
}
}
to android/defaultConfig in your's build.gradle and a JSON with complete schemas will be generated.
To do these migrations safely you can fully test them - this is well described here
Regarding efficiency I would recommend creating direct migrations like Migration(1, 4)

The android room migrations are problematic, because even a difference of column order will show a failed migration. But due to sqlite's limitations column type alterations and so forth cannot be done. Do as in Michal's answer and generate the schemas.
Then use a JSON diff tool to see how it generates your particular table.
If the difference is easy, do the necessary alter command in your migration script. I've pretty much given up on doing this and instead just create a new table the way my new version expects it to be, copy the data from the old table, drop it, and rename the correctly created table. Here's some code:
static final Migration MIGRATION_3_4 = new Migration(3, 4) {
#Override
/**
* Instead of trying to read the owl droppings of the error message, simply go to the schemas directory
* and do a table copy of the before and after. Look for the table creation in the updated schema number
* then do an insert by doing a select * from the old
* then remove the old and rename the new
*/
public void migrate(SupportSQLiteDatabase _db) {
_db.execSQL("CREATE TABLE IF NOT EXISTS crew2 (`crewId` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `name` TEXT NOT NULL, `shortName` TEXT, `isCaptain` TEXT)");
_db.execSQL("INSERT INTO crew2(crewid,name,shortname) SELECT * FROM crew_table");
_db.execSQL("DROP TABLE crew_table");
_db.execSQL("ALTER TABLE crew2 RENAME TO crew_table");
_db.execSQL("CREATE UNIQUE INDEX IF NOT EXISTS `index_crew_table_shortName` ON `crew_table` (`shortName`)");
}
};

How do I completely recreate my database in Android Room?

I have a situation where I want to be able to do a hard reset of my database using Android Room. Using SQLiteOpenHelper, I could do this by writing a method to drop all tables and indices, then manually call SQLiteOpenHelper.onCreate().
I'm aware that I can get access to a SupportSQLiteOpenHelper via room and drop all the tables myself, but there doesn't seem to be a good way to kick-off the recreation process.
Also, I'm aware that I could delete every item from each table without dropping it, but that's not what I want. That doesn't reset the auto-incrementing primary key, so the "id" field of new items won't reset back to 1.
Thanks!
EDIT:
This is something I want to be able to do arbitrarily at runtime.
EDIT 2:
The method should be maintainable, i.e. not involve hand-writing SQL that matches Room's behavior. Ideally there would be some way to retrieve the SQL that Room generates, or a SQLiteOpenHelper.onCreate() equivalent method. Or anything else that solves this problem! :)

I found it easiest to do this via context.deleteDatabase(“name”) and then simply reinstantiating and repopulating the database via the Room.databaseBuilder().addCallback upon first access.

Simple answer is to increment your #Database version number. Generally speaking you should do this for schema changes, however it will achieve your aims of clearing the database and resetting all primary key values.
#Database(entities = { Hello.class }}, version = 1) // <- you want to increase version by 1
#TypeConverters({})
public abstract class AppDatabase extends RoomDatabase {
public abstract HelloDao helloTextDao();
}
EDIT: IF you want to do this at run time, I would clear all the data from your tables (to avoid FK issues), then call DROP TABLE table_name, on all of your respective tables. Then you will need to write queries for creating the tables ie : CREATE TABLE table_name (uid INTEGER PRIMARY KEY, name STRING);.
This will mean you have to maintain a list of create table queries up to date with your POJO's unfortunatly. Ideally you'd be able to use reflection to generate the query, howerever #ColumnInfo currently doesn't support reflection as it has its RetentionPolicy set to CLASS.
Hopefully this solves your problem, feel free to ask for further clarification in the comments. Happy hunting!

For my case, deleting database with context.deleteDatabase(“name”) still caches the data of the old database.
I solved this by manually deleting the database file
File databasesDir = new File(context.getApplicationInfo().dataDir + "/databases");
new File(databasesDir, "MyDatabaseName.db").delete();

I was not able to find a way to do this programatically.
But, you can go to yourApp -> Long click -> App Info -> Storage & Cache -> clear both cache and Storage.
Clearing storage displays a verification dialog that indicates that databases will be destroyed.

This may help you.
upgrade your Database version
When you call DatabaseBuilder, remember add this to your code fallbackToDestructiveMigration
Room.databaseBuilder(context, Database::class.java, DB_NAME)
.allowMainThreadQueries()
.fallbackToDestructiveMigration()
.build()
More detail please refer this

As per the answers given here, and here, and here, I could do it as following:
public void deleteAndReset() {
SQLiteDatabase database;
database = SQLiteDatabase.openOrCreateDatabase(context.getDatabasePath(MY_DATABASE_NAME), null);
String deleteTable = "DELETE FROM " + MY_TABLE_NAME;
database.execSQL(deleteTable);
String deleteSqliteSequence = "DELETE FROM sqlite_sequence WHERE name = '" + MY_TABLE_NAME + "'";
database.execSQL(deleteSqliteSequence);
}

Android SQLite Database Update

I have a pre-established SQLite Database in my application. It has a table with rows about 20 rows of text. I want to be able to add additional rows to the table without deleting all of the previous information. The only way I have seen which would allow me to do this is to delete all of the previous databases and then recreate it with the new rows. There must be a better way. Thanks for your help!

Are you confusing rows with columns?
If you really do mean rows then as antlersoft points out, using the SQL INSERT INTO statement will simply add a new row to a table without affecting any existing table data. This is one of the most basic and commonly used SQL statements.
If you actually mean you need to add columns then use the SQL ALTER TABLE statement.
See..
SQL INSERT INTO statement
SQL ALTER TABLE statement

The Android framework, as it relates to SQLite (using a SQLiteOpenHelper) provides two distinct methods for handling database lifecycles - onCreate(), used when the database needs to be created from scratch, and onUpgrade(<database>, int oldVersion, int newVersion) for handling updates. You can specify the "new" version number in the constructor for the superclass of your SQLiteOpenHelper, and the framework knows to call onUpgrade() based on this parameter and the internal version # in the actual sqlite database.
So, to modify your database during a version change just override onUpgrade() and run whatever SQLite stuff that you need.