什么是初始迁移，在现有应用程序中设置 Room (SQLite)？

Question

我正致力于在我的应用程序中设置一个小型 SQLite 数据库，该数据库到目前为止一直将数据存储在 OS MediaStore 中。我想远离它，并使用 Room。

最初的table非常简单，基本上只是一个用于描述的TEXT字段，以及MediaStore中实际图像的一些ID。但是，我也想设置其他几个 table。这看起来有点合理吗：

import android.provider.BaseColumns;
import android.provider.MediaStore;

public class AlbumContract {

    public static class PhotoEntry implements BaseColumns {
        public static final String TABLE_NAME = "photo";
        public static final String COLUMN_NAME_IMAGE_ID = "fk_image_id";
        public static final String COLUMN_NAME_IMAGE_URI = "mediastore_uri";
        public static final String COLUMN_NAME_EVENT_ID = "fk_event_id";
        public static final String COLUMN_NAME_STACK_ID = "fk_stack_id";
        public static final String COLUMN_NAME_DESCRIPTION = "description";
    }

    public static class StackEntry implements BaseColumns {
        public static final String TABLE_NAME = "stack";
        public static final String COLUMN_NAME_STACK_LEADER = "fk_photo_id";
    }

    public static class EventEntry implements BaseColumns {
        public static final String TABLE_NAME = "event";
        public static final String COLUMN_NAME_TITLE = "title";
        public static final String COLUMN_NAME_SUBTITLE = "subtitle";
        public static final String COLUMN_NAME_ALBUM_ID = "fk_album_id";
    }

    public static class AlbumEntry implements BaseColumns {
        public static final String TABLE_NAME = "album";
        public static final String COLUMN_NAME_TITLE = "title";
        public static final String COLUMN_NAME_SUBTITLE = "subtitle";
    }

    public static final String SQL_CREATE_PHOTO =
            "CREATE TABLE " + PhotoEntry.TABLE_NAME + " (" +
                    PhotoEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                    PhotoEntry.COLUMN_NAME_IMAGE_ID + " INTEGER, " +
                    PhotoEntry.COLUMN_NAME_IMAGE_URI + " TEXT, " +
                    PhotoEntry.COLUMN_NAME_STACK_ID + " INTEGER, " +
                    PhotoEntry.COLUMN_NAME_EVENT_ID + " INTEGER, " +
                    PhotoEntry.COLUMN_NAME_DESCRIPTION + " TEXT, " +
                    // Foreign key to MediaStore.Images
                    "FOREIGN KEY (" + PhotoEntry.COLUMN_NAME_IMAGE_ID + ") " +
                    "REFERENCES " + MediaStore.Images.Media.EXTERNAL_CONTENT_URI + "(" + MediaStore.Images.Media._ID + ") " +
                    // Foreign key to "stack" table
                    "FOREIGN KEY (" + PhotoEntry.COLUMN_NAME_STACK_ID + ") " +
                    "REFERENCES " + StackEntry.TABLE_NAME + "(" + StackEntry._ID + ") " +
                    // Foreign key to "event" table
                    "FOREIGN KEY (" + PhotoEntry.COLUMN_NAME_EVENT_ID + ") " +
                    "REFERENCES " + EventEntry.TABLE_NAME + "(" + EventEntry._ID + ") " +
                    ")";

    public static final String SQL_CREATE_STACK =
            "CREATE TABLE " + StackEntry.TABLE_NAME + " (" +
                    StackEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                    StackEntry.COLUMN_NAME_STACK_LEADER + " INTEGER " +
                    "FOREIGN KEY (" + StackEntry.COLUMN_NAME_STACK_LEADER + ") " +
                    "REFERENCES " + PhotoEntry.TABLE_NAME + "(" + PhotoEntry._ID + ") " +
                    ")";

    public static final String SQL_CREATE_EVENT =
            "CREATE TABLE " + EventEntry.TABLE_NAME + " (" +
                    EventEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                    EventEntry.COLUMN_NAME_ALBUM_ID + " INTEGER, " +
                    EventEntry.COLUMN_NAME_TITLE + " TEXT, " +
                    EventEntry.COLUMN_NAME_SUBTITLE + " TEXT, " +
                    "FOREIGN KEY (" + EventEntry.COLUMN_NAME_ALBUM_ID + ") " +
                    "REFERENCES " + AlbumEntry.TABLE_NAME + "(" + AlbumEntry._ID + ") " +
                    ")";

    public static final String SQL_CREATE_ALBUM =
            "CREATE TABLE " + AlbumEntry.TABLE_NAME + " (" +
                    AlbumEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                    AlbumEntry.COLUMN_NAME_TITLE + " TEXT, " +
                    AlbumEntry.COLUMN_NAME_SUBTITLE + " TEXT)";
}

该应用程序允许用户将 captions/descriptions 存储到设备上的图像中，因此如果用户对图像进行注释，则图像 ID + 标题将存储在 photo table .与 stack table 存在 1:n 关系，其中多个图像可以是 "stack" 的一部分。此外，照片序列（严格假设 increasing/consecutive ID）可以分组为 event。并且可以将多个事件分组为 album。我正在尝试使用外键约束将其形式化。

但我想从小处着手，仅使用 photo table 并让它发挥作用 :) 我的问题是：如何通过迁移在正确的方向下车，如onUpgrade() 等等？我想第一次迁移是从没有数据库到设置数据库？

public class MyappDbHelper extends SQLiteOpenHelper {
    // If you change the database schema, you must increment the database version.
    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "Myapp.db";

    public MyappDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
    public void onCreate(SQLiteDatabase db) {
        //db.execSQL(AlbumContract.SQL_CREATE_ALBUM);  // I'm starting with just the PHOTO and STACK tables
        //db.execSQL(AlbumContract.SQL_CREATE_EVENT);
        db.execSQL(AlbumContract.SQL_CREATE_PHOTO);
        db.execSQL(AlbumContract.SQL_CREATE_STACK);
    }
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        //db.execSQL(SQL_DELETE_ENTRIES);
        onCreate(db);
    }
    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        onUpgrade(db, oldVersion, newVersion);
    }
}

Answer 1

看来您正在尝试使用旧方法来创建 SQLite 数据库。如果您使用 Room 库来处理 SQLite，那么您必须为您的数据创建 @Entity 和 @Dao（数据访问对象）classes，并且 @Database class 扩展 RoomDatabase.

您的案例代码如下所示：

@Entity
public class Photo {

    @PrimaryKey
    public int photoId;

    @ColumnInfo(name = "mediastoreUri")
    public String mediastoreUri;

    @ColumnInfo(name = "description")
    public String description;

    @ColumnInfo(name = "photoEventId")
    public long photoEventId;
}

@Entity
public class Event {

    @PrimaryKey
    public int eventId;

    @ColumnInfo(name = "title")
    public String title;

    @ColumnInfo(name = "subtitle")
    public String subtitle;
}

// One-to-Many relationship
public class EventWithPhotos {

    @Embedded
    public Event event;

    @Relation(
        parentColumn = "eventId",
        entityColumn = "photoEventId"
    )
    public List<Photo> photos;
}

@Dao
public interface PhotoDao {

    @Query("SELECT * FROM Photo")
    List<Photo> getAll();

    @Query("SELECT * FROM Photo WHERE photoId IN (:ids)")
    List<Photo> loadAllByIds(int[] ids);

    @Insert
    void insertAll(Photo... photos);

    @Delete
    void delete(Photo photo);
}

@Database(entities = {Photo.class, Event.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
    public abstract PhotoDao userDao();
}

有关详细信息，请参阅：Room overview, Relationships in Room, Migrations in Room

什么是初始迁移，在现有应用程序中设置 Room (SQLite)？

What's the initial migration, setting up Room (SQLite) in an existing app?

android

android-sqlite

android-room