房间的存在

房间需要四个部分：数据库类，DAO 类，实体类和迁移类（现在你可以只使用 DDL 方法 ）：

实体类

// Set custom table name, add indexes
@Entity(tableName = "videos",
        indices = {@Index("title")}
)
public final class VideoItem {
    @PrimaryKey // required
    public long articleId;
    public String title;
    public String url;
}

// Use ForeignKey for setup table relation
@Entity(tableName = "tags",
        indices = {@Index("score"), @Index("videoId"), @Index("value")},
        foreignKeys = @ForeignKey(entity = VideoItem.class,
                parentColumns = "articleId",
                childColumns = "videoId",
                onDelete = ForeignKey.CASCADE)
)
public final class VideoTag {
    @PrimaryKey
    public long id;
    public long videoId;
    public String displayName;
    public String value;
    public double score;
}

DAO 类

@Dao
public interface VideoDao {
    // Create insert with custom conflict strategy
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void saveVideos(List<VideoItem> videos);

    // Simple update
    @Update
    void updateVideos(VideoItem... videos);

    @Query("DELETE FROM tags WHERE videoId = :videoId")
    void deleteTagsByVideoId(long videoId);

    // Custom query, you may use select/delete here
    @Query("SELECT v.* FROM tags t LEFT JOIN videos v ON v.articleId = t.videoId WHERE t.value = :tag ORDER BY updatedAt DESC LIMIT :limit")
    LiveData<List<VideoItem>> getVideosByTag(String tag, int limit);
}

数据库类

// register your entities and DAOs
@Database(entities = {VideoItem.class, VideoTag.class}, version = 2)
public abstract class ContentDatabase extends RoomDatabase {
    public abstract VideoDao videoDao();
}

迁移

public final class Migrations {
    private static final Migration MIGRATION_1_2 = new Migration(1, 2) {
        @Override
        public void migrate(SupportSQLiteDatabase database) {
            final String[] sqlQueries = {
                    "CREATE TABLE IF NOT EXISTS `tags` (`id` INTEGER PRIMARY KEY AUTOINCREMENT," +
                            " `videoId` INTEGER, `displayName` TEXT, `value` TEXT, `score` REAL," +
                            " FOREIGN KEY(`videoId`) REFERENCES `videos`(`articleId`)" +
                            " ON UPDATE NO ACTION ON DELETE CASCADE )",
                    "CREATE  INDEX `index_tags_score` ON `tags` (`score`)",
                    "CREATE  INDEX `index_tags_videoId` ON `tags` (`videoId`)"};
            for (String query : sqlQueries) {
                database.execSQL(query);
            }
        }
    };

    public static final Migration[] ALL = {MIGRATION_1_2};

    private Migrations() {
    }
}

在 Application 类中使用或通过 Dagger 提供

ContentDatabase provideContentDatabase() {
    return Room.databaseBuilder(context, ContentDatabase.class, "data.db")
            .addMigrations(Migrations.ALL).build();
}

编写你的存储库：

public final class ContentRepository {
    private final ContentDatabase db;
    private final VideoDao videoDao;

    public ContentRepository(ContentDatabase contentDatabase, VideoDao videoDao) {
        this.db = contentDatabase;
        this.videoDao = videoDao;
    }

    public LiveData<List<VideoItem>> getVideoByTag(@Nullable String tag, int limit) {
        // you may fetch from network, save to database
        ....
        return videoDao.getVideosByTag(tag, limit);
    }
}

在 ViewModel 中使用：

ContentRepository contentRepository = ...;
contentRepository.getVideoByTag(tag, limit);