android - 在现有应用程序中设置 Room (SQLite) 的初始迁移是什么?
问题描述
我正在我的应用程序中设置一个小型 SQLite 数据库,到目前为止,它一直将数据存储在 OS MediaStore 中。我想摆脱它,并使用 Room。
初始表很简单,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表。这看起来有点明智:
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在设备上存储图像的标题/描述,因此如果用户注释图像,则图像 ID + 标题将存储在photo
表中。与表格存在 1:n 关系stack
,其中多个图像可以是“堆栈”的一部分。此外,照片序列(假设严格增加/连续的 ID)可以分组为event
. 并且可以将多个事件分组为一个album
. 我正在尝试使用外键约束将其形式化。
但是我想从小处着手,只用photo
桌子并让它发挥作用:) 我的问题是:我如何在迁移中正确地下车,例如onUpgrade()
等?我猜第一次迁移是从没有数据库到设置一个数据库?
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);
}
}
解决方案
您似乎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)类,以及@Database
扩展RoomDatabase
.
对于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();
}
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