java - 如何在 Room 2.3.0 中向我的实体添加外键？

Question

我有一个使用 Room 2.2.5 的项目，我刚刚更新到 2.3.0 版本，这是一个名为 photo 的实体的代码:

    @Entity(tableName = Photo.TABLE_NAME, foreignKeys = @ForeignKey(entity = Event.class,
        parentColumns = "_id",
        childColumns = "id_event_ft",
        onDelete = ForeignKey.CASCADE))
public class Photo {

    public static final String TABLE_NAME = "Photo";
    public static final String COLUMN_ID = BaseColumns._ID;

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(index = true, name = COLUMN_ID)
    public Long id_photo;

    @ColumnInfo(name = "path")
    private String path;

    @ForeignKey(entity = Event.class,
            parentColumns = "_id",
            childColumns = "id_event_ft",
            onDelete = ForeignKey.CASCADE)
    private Long id_event_ft;

    public Photo(Long id_photo, String path, Long id_event_ft) {
        this.id_photo = id_photo;
        this.path = path;
        this.id_event_ft = id_event_ft;
    }

    public Long getId_photo() {
        return id_photo;
    }

    public void setId_photo(Long id_photo) {
        this.id_photo = id_photo;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public Long getId_event_ft() {
        return id_event_ft;
    }

    public void setId_event_ft(Long id_event_ft) {
        this.id_event_ft = id_event_ft;
    }
}

现在我在尝试编译时收到以下错误

error: annotation type not applicable to this kind of declaration@ForeignKey(entity = Event.class, parentColumns = "_id", childColumns = "id_event_ft", onDelete = ForeignKey.CASCADE)^

错误出现在变量 private Long id_event_ft; 上方的 @ForeignKey 中。
在 documentation我找到了这个:

Added missing target to @ForeignKey annotation preventing its usage outside of the @Entity annotation. (Iced1e)

很明显，不再允许在 @Entity 注释之外使用 @ForeignKey，但是我该如何绑定(bind) id_event_ft外键的变量？，我现在如何为其赋值？
我希望有人可以帮助我，非常感谢

Answer 1

使用 外键 不会自动(神奇地)建立关系。相反，它允许主要通过强制引用完整性来支持关系。

外键(关系)的存在不需要外键定义。

也就是说，它定义了一个规则，即子列的值 ( id_event_ft ) 必须是父列中存在的值 ( _id )。如果存在外键冲突，它还支持处理(例如，您使用的 onDelete)。
实际上，提供合适的值是您必须以编程方式执行的操作，即 id 添加照片，您必须确定照片要链接/相关的事件。
您可以使用@Relation 来简化提取相关数据的过程。
所以考虑以下几点:
一个 Activity 实体(很好，很简单的演示)

@Entity
public class Event {
    @PrimaryKey(autoGenerate = true)
    Long _id = null;
    String other_columns;

    public Event(){}
    @Ignore
    public Event(String other_columns) {
        this.other_columns = other_columns;
    }
}

照片的父列将是 _id 柱子。

Second Ignored (i.e. Ignored by Room) 构造函数，否则 Room 会发出警告，如 *warning: There are multiple good constructors and Room 将选择无参数构造函数。 *

略有改动 照片 实体 :-

@Entity(tableName = Photo.TABLE_NAME,
        foreignKeys = @ForeignKey(
                entity = Event.class,
                parentColumns = "_id",
                childColumns = "id_event_ft",
                onDelete = ForeignKey.CASCADE),
        indices = @Index("id_event_ft") //<<<<<<<<<< ADDED as Room warns if omitted
)
public class Photo {

    public static final String TABLE_NAME = "Photo";
    public static final String COLUMN_ID = BaseColumns._ID;

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(index = true, name = COLUMN_ID)
    public Long id_photo;
    @ColumnInfo(name = "path")
    private String path;
    /* <<<<<<<< COMMENTED OUT >>>>>>>>>>
    @ForeignKey(entity = Event.class,
            parentColumns = "_id",
            childColumns = "id_event_ft",
            onDelete = ForeignKey.CASCADE)

     */
    private Long id_event_ft;

    public Photo(Long id_photo, String path, Long id_event_ft) {
        this.id_photo = id_photo;
        this.path = path;
        this.id_event_ft = id_event_ft;
    }

    public Long getId_photo() {
        return id_photo;
    }

    public void setId_photo(Long id_photo) {
        this.id_photo = id_photo;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public Long getId_event_ft() {
        return id_event_ft;
    }

    public void setId_event_ft(Long id_event_ft) {
        this.id_event_ft = id_event_ft;
    }
}

为了演示通过关系检索 POJO EventWithPhotos :-

public class EventWithPhotos {

    @Embedded
    Event event;
    @Relation(entity = Photo.class,parentColumn = "_id",entityColumn = "id_event_ft")
    List<Photo> photos;
}

现道 全道 :-

@Dao
interface AllDao {

    @Insert
    long insert(Event event);
    @Insert
    long insert(Photo photo);
    @Transaction
    @Query("SELECT * FROM event")
    List<EventWithPhotos> getAllEventsWithPhotos();
}

How do I assign a value to it now?

现在是一个将所有事件放在一起的示例，第一个事件有 2 张照片，第二个事件有 1 张照片。请注意使用的不同技术:-

    dao = db.getAllDao();

    // Prepare to add an Event
    Event newEvent = new Event();
    newEvent.other_columns = "Event1";
    // Add the Event retrieving the id (_id column)
    long eventId = dao.insert(newEvent);
    // Prepare a photo to be added to Event1
    Photo newPhoto = new Photo(null,"photo1",eventId);
    // Add the Photo to Event1
    long photoid = dao.insert(newPhoto);
    // Add second photo to Event 1 using the 2nd constructor
    dao.insert(new Photo(null,"photo2",eventId));
    // Add Event2 with a photo all in a single line (again using the 2nd constrcutor)
    long event2Id;
    dao.insert(new Photo(null,"photo3",event2Id = dao.insert(new Event("Event2"))));

    // Get and output Each Event with the Photos for that Event
    List<EventWithPhotos> allEventsWithPhotosList = dao.getAllEventsWithPhotos();
    for (EventWithPhotos ewp: allEventsWithPhotosList) {
        Log.d("EVENTPHOTOINFO","Event is " + ewp.event.other_columns);
        for (Photo p: ewp.photos) {
            Log.d("EVENTWITHPHOTO","\tPhoto is " + p.getPath() + " ID is " + p.getId_photo());
        }
    }

结果
运行时日志包含:-

D/EVENTPHOTOINFO: Event is Event1
D/EVENTWITHPHOTO:   Photo is photo1 ID is 1
D/EVENTWITHPHOTO:   Photo is photo2 ID is 2
D/EVENTPHOTOINFO: Event is Event2
D/EVENTWITHPHOTO:   Photo is photo3 ID is 3

数据库(使用数据库检查器查看)显示:-
Activity table :-

照片 table :-