Room 中的多态实体 [英] Polymorphic entities in Room
问题描述
我的 Room DB 中有 3 个实体:
There are 3 entities in my Room DB:
相册、PhotosMediaItem和VideosMediaItem.
VideosMediaItem 和 PhotosMediaItem 继承自 MediaItem.
MediaItem 不是数据库中的实体，它只是一个抽象基类.
MediaItem is not an entity in the DB, it's just an abstract base class.
我想创建一个查询，以根据创建日期降序返回特定相册中的所有照片和视频媒体项.
I would like to create a query that returns all the photos and videos media items in a specific album with descending order based on their creation date.
因此查询将创建一个包含派生类型的 MediaItem 列表.(PhotoMediaItem 或 VideoMediaItem) 以多态方式.
So the query will create a list of MediaItems but with the derived types. (PhotoMediaItem or VideoMediaItem) in a polymorphic way.
这是我尝试过的:
@Query("SELECT * FROM PhotosMediaItem WHERE PhotosMediaItem = :albumId " +
"UNION SELECT * FROM VideosMediaItem WHERE VideosMediaItem = :albumId" +
" ORDER by CreationDate DESC")
List<MediaItem> getAllMediaInAlbum(int albumId);
这显然不起作用，因为它试图启动 MediaItem 对象，这不是我的意图.我希望此查询启动派生类 PhotoMediaItem 或 VideoMediaItem
This won't work obviously, because it tries to initiate MediaItem object, and it is not my intention. I want this query to initiate the derived class, PhotoMediaItem or VideoMediaItem
这是我的查询在迁移到 Room 之前的样子，使用常规 SQLiteHelper，它工作得很好:
Here's how my query looked like before the migration to Room, using the regular SQLiteHelper, and it worked just fine:
public ArrayList<MediaItem> getMediaListByAlbumId(int palbumId)
{
Cursor cursor = null;
try{
ArrayList<MediaItem> mediaList = new ArrayList<>();
String selectQuery = "SELECT "+ mPhotoId +","+ mPhotoCreationDate +", 0 AS mediaType, '' FROM "+ mPhotosTableName + " WHERE " + this.mPhotoAlbumId + "="+palbumId +
" UNION " +
"SELECT "+ mVideoId +","+ mVideoCreationDate + " ,1 AS mediaType, " + mVideoLength + " FROM " + mVideosTableName + " WHERE " + this.mVideoAlbumId +"="+palbumId +
" ORDER BY CreationDate DESC";
cursor = mDB.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()){
do {
// MediaHolder consists of the media ID and its type
int mediaType = cursor.getInt(2);
MediaItem mediaItem = null;
if (mediaType == 0) {
mediaItem = new PhotoMediaItem(cursor.getInt(0), null, palbumId);
} else if (mediaType == 1) {
mediaItem = new VideoMediaItem(cursor.getInt(0), null, palbumId, cursor.getLong(3));
}
mediaList.add(mediaItem);
}
while (cursor.moveToNext());
}
return mediaList;
}
finally {
if(cursor != null){
cursor.close();
}
}
}
那么如何使用 Room 达到同样的效果呢?
How can I achieve the same effect using Room then?
推荐答案
我认为你有很多选择:
选项 1
您使用单个表来存储所有 MediaItem，并使用鉴别器列来区分视频和照片.您有一个执行查询、应用 order by 并返回 Cursor 的单一 DAO 方法.然后，您可以使用现有的光标操作逻辑返回 List<MediaItem>它可能看起来像这样:
You use a single table to store all MediaItems and you use a discriminator column to make the difference between a video and a photo.
You have a single DAO method that performs the query, applies the order by and returns a Cursor. Then you can use your existing cursor manipulation logic to return a List<MediaItem>
It can look like this:
@Dao
public abstract class MediaItemDao() {
@Query("you query here")
protected Cursor getByAlbumIdInternal(int albumId);
public List<MediaItem> getByAbumId(int albumId) {
Cursor cursor = null;
try{
List<MediaItem> mediaList = new ArrayList<>();
cursor = getByAlbumIdInternal(albumId);
// looping through all rows and adding to list
if (cursor.moveToFirst()){
do {
// use the discriminator value here
int mediaType = cursor.getInt(cursor.getColumnIndex("you discriminator column name here"));
MediaItem mediaItem = null;
if (mediaType == 0) {
mediaItem = new PhotoMediaItem(cursor.getInt(0), null, palbumId);
} else if (mediaType == 1) {
mediaItem = new VideoMediaItem(cursor.getInt(0), null, palbumId, cursor.getLong(3));
}
mediaList.add(mediaItem);
} while (cursor.moveToNext());
}
return mediaList;
}
finally {
if(cursor != null){
cursor.close();
}
}
}
}
选项 2
您使用两个不同的表来存储 VideosMediaItem 和 PhotosMediaItem.您有一个 MediaItemDao，它有两个执行查询的内部方法和一个将两个结果集合并在一起并在 java 代码中应用排序的公共方法.它可能看起来像这样:
You use two different tables to store VideosMediaItem and PhotosMediaItem. You have a MediaItemDao that has two internal methods to perform the queries and a single public method that merges the two result sets together and applies the sorting in java code. It can look like this:
@Dao
public abstract class MediaItemDao() {
@Query("your query to get the videos, no order by")
protected List<VideoMediaItem> getVideosByAlbumId(int albumId);
@Query("your query to get the photos, no order by")
protected List<PhotosMediaItem> getPhotosByAlbumId(int albumId);
@Transaction
public List<MediaItem> getByAlbumId(int albumId) {
final List<MediaItem> mediaItems = new LinkedList<>();
mediaItems.add(getVideosByAlbumId(albumId));
mediaItems.add(getPhotosByAlbumId(albumId));
Collections.sort(mediaItems, <you can add a comparator here>);
return mediaItems;
}
}
如何为此选项利用实时数据?
正如我所提到的，您应该使用 LiveData 作为受保护方法的返回类型，以便在这些表的基础更改时收到通知.所以它们应该是这样的:
As I mentioned, you should use LiveData as the return type of your protected methods so you get notified for underlying changes on those tables. So they should look like this:
protected LiveData<List<VideoMediaItem>> getVideosByAlbumId(int albumId);
protected LiveData<List<PhotosMediaItem>> getPhotosByAlbumId(int albumId);
为了向客户端返回单个 LiveData，您应该将这两个方法的输出压缩到单个流中.您可以使用自定义 MediatorLiveData 实现来实现此目的.它可能看起来像这样:
In order to return a single LiveData to the client, you should zip the outputs of those two methods into a single stream. You can achieve this using a custom MediatorLiveData implementation. It may look like this:
public class ZipLiveData<T1, T2, R> extends MediatorLiveData<R> {
private T1 mLastLeft;
private T2 mLastRight;
private Zipper<T1, T2, R> mZipper;
public static final <T1, T2, R> LiveData<R> create(@NonNull LiveData<T1> left, @NonNull LiveData<T2> right, Zipper<T1, T2, R> zipper) {
final ZipLiveData<T1, T2, R> liveData = new ZipLiveData(zipper);
liveData.addSource(left, value -> {
liveData.mLastLeft = value;
update();
});
liveData.addSource(right, value -> {
liveData.mLastRight = value;
update();
});
return liveData;
}
private ZipLiveData(@NonNull Zipper<T1, T2, R> zipper) {
mZipper = zipper;
}
private update() {
final R result = zipper.zip(mLastLeft, mLastRight);
setValue(result);
}
public interface Zipper<T1, T2, R> {
R zip(T1 left, T2 right);
}
}
然后您只需在存储库公共方法中使用它，如下所示:
Then you just use it in your repository public method like this:
public List<MediaItem> getByAlbumId(int albumId) {
return ZipLiveData.create(
getVideosByAlbumId(albumId),
getPhotosByAlbumId(albumId),
(videos, photos) -> {
final List<MediaItem> mediaItems = new LinkedList<>();
mediaItems.add(videos);
mediaItems.add(photos);
Collections.sort(mediaItems, <you can add a comparator here>);
return mediaItems;
}
}
选项 3
这仅适用于您有存储库层的情况.
This is applicable only if you have a repository layer in place.
您使用两个不同的表来存储 VideosMediaItem 和 PhotosMediaItem.每个都有一个 DAO 类.您有一个依赖于两个 DAO 并组合结果集并应用排序的存储库.它可能看起来像这样:
You use two different tables to store VideosMediaItem and PhotosMediaItem. You have a DAO class for each one. You have a repository that depends on the both DAOs and combines the result sets, applying the sorting. It can look like this:
@Dao
public abstract class VideosMediaItemDao {
@Query("your query to get the videos, no order by")
public List<VideoMediaItem> getByAlbumId(int albumId);
}
@Dao
public abstract class PhotosMediaItemDao {
@Query("your query to get the photos, no order by")
public List<PhotosMediaItem> getByAlbymId(int albumId);
}
public interface MediaItemRepository {
public List<MediaItem> getByAlbumId(int albumId);
}
class MediaItemRepositoryImpl {
private final VideosMediaItemDao mVideoDao;
private final PhotosMediaItemDao mPhotoDao;
MediaItemRepositoryImpl(VideosMediaItemDao videoDao, PhotosMediaItemDao photoDao) {
mVideoDao = videoDao;
mPhotoDao = photoDao;
}
@Override
public List<MediaItem> getByAlbumId(int albumId) {
final List<MediaItem> mediaItems = new LinkedList<>();
mediaItems.add(mVideoDao.getByAlbumId(albumId));
mediaItems.add(mPhotoDao.getByAlbumId(albumId));
Collections.sort(mediaItems, <you can add a comparator here>);
return mediaItems;
}
}
这篇关于Room 中的多态实体的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
