如何在 Room 2.3.0 中为我的实体添加外键？

Question

我有一个使用 Room 2.2.5 的项目，我刚刚更新到版本 2.3.0 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遇到以下错误

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

错误在变量上方的@ForeignKey中private Long id_event_ft;

在 documentation 我发现了这个：

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

显然不再允许在@Entity 注释之外使用@ForeignKey，但是如何将id_event_ft 变量绑定到外键？，如何为其赋值现在？

希望有人能帮帮我，非常感谢

Answer 1

关系

@多对一

如果关系是一个事件的多张照片，请使用 ORM Hibernate，这是在照片实体上定义外键约束的最简单方法

@ManyToOne
@JoinColumn(name = "id", nullable = false)
private Event id_event_ft;

Answer 2

使用 ForeignKey 不会自动（神奇地）建立关系。相反，它允许主要通过强制参照完整性来支持关系。

外键（关系）不需要外键定义。

它定义了一条规则，规定子列 (id_event_ft) 的值必须是存在于父列 ( _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 suitable 值是您必须以编程方式执行的操作，即添加照片的 id，您必须确定照片要 linked/related 到哪个事件。

您可以使用@Relation 来简化相关数据的提取。

所以请考虑以下几点：

一个事件实体（对于演示来说很简单）

@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 列。
第二个忽略（即被 Room 忽略）的构造函数，否则 Room 会发出警告，如 *warning: There are multiple good constructors and Room will pick the no-arg constructor. *

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;
    }
}

用于演示通过关系检索 POJO EventWithPhotos :-

public class EventWithPhotos {

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

Now a Dao AllDao:-

@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?

现在举个例子，将所有内容放在一起，添加 2 个事件，第一个事件有 2 张照片，第二个事件有 1 照片。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

当运行日志包含：-

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：-

事件table:-

照片table:-

如何在 Room 2.3.0 中为我的实体添加外键？

How do I add a foreign key to my entity in Room 2.3.0?

java

sqlite

android

android-room

关系

@多对一