Database

public abstract @interface Database
implements Annotation

android.arch.persistence.room.Database

Marks a class as a RoomDatabase.

The class should be an abstract class and extend RoomDatabase.

You can receive an implementation of the class via Room.databaseBuilder or Room.inMemoryDatabaseBuilder. 

 // User and Book are classes annotated with @Entity.
 @Database(version = 1, entities = {User.class, Book.class})
 abstract class AppDatabase extends RoomDatabase {
     // BookDao is a class annotated with @Dao.
     abstract public BookDao bookDao();
     // UserDao is a class annotated with @Dao.
     abstract public UserDao userDao();
     // UserBookDao is a class annotated with @Dao.
     abstract public UserBookDao userBookDao();
 }
The example above defines a class that has 2 tables and 3 DAO classes that are used to access it. There is no limit on the number of Entity or Dao classes but they must be unique within the Database.

Instead of running queries on the database directly, you are highly recommended to create Dao classes. Using Dao classes will allow you to abstract the database communication in a more logical layer which will be much easier to mock in tests (compared to running direct sql queries). It also automatically does the conversion from Cursor to your application classes so you don't need to deal with lower level database APIs for most of your data access.

Room also verifies all of your queries in Dao classes while the application is being compiled so that if there is a problem in one of the queries, you will be notified instantly.

Summary

Public methods

Class[] entities()

The list of entities included in the database.

boolean exportSchema()

You can set annotation processor argument (room.schemaLocation) to tell Room to export the schema into a folder.

int version()

The database version.

Inherited methods

Public methods

entities

Class[] entities ()

The list of entities included in the database. Each entity turns into a table in the database.

Returns
Class[] The list of entities in the database.

exportSchema

boolean exportSchema ()

You can set annotation processor argument (room.schemaLocation) to tell Room to export the schema into a folder. Even though it is not mandatory, it is a good practice to have version history in your codebase and you should commit that file into your version control system (but don't ship it with your app!).

When room.schemaLocation is set, Room will check this variable and if it is set to true, its schema will be exported into the given folder.

exportSchema is true by default but you can disable it for databases when you don't want to keep history of versions (like an in-memory only database).

Returns
boolean Whether the schema should be exported to the given folder when the room.schemaLocation argument is set. Defaults to true.

version

int version ()

The database version.

Returns
int The database version.