Java 如何向房间2.3.0中的实体添加外键？
我有一个项目，我与Room 2.2.5合作，我刚刚更新到版本2.3.0这是一个名为photo的实体的代码：Java 如何向房间2.3.0中的实体添加外键？,java,android,sqlite,android-room,Java,Android,Sqlite,Android Room,我有一个项目，我与Room 2.2.5合作，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
@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;
}
}
private Long id\u event\u ft上方的@ForeignKey中
在这本书中，我发现：
将缺少的目标添加到@ForeignKey注释中，以防止其在@Entity注释之外使用。（Iced1e）
显然，不再允许在@Entity注释之外使用@ForeignKey，但是如何将id\u event\u ft
变量绑定到外键？现在如何为其赋值
我希望有人能帮助我，非常感谢你
@多对一
如果一个事件的关系是多张照片，请使用ORM Hibernate，这是定义带有照片实体约束的外键的最简单方法
@ManyToOne
@JoinColumn(name = "id", nullable = false)
private Event id_event_ft;
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）不需要外键定义
也就是说，它定义了一条规则，规定子列（id\u event\u ft）的值必须是父列（\u id）中存在的值。它还支持处理外键冲突（例如，您使用的onDelete
）
实际上，提供一个合适的值是您必须以编程方式完成的事情，即添加一张照片，您必须确定照片将链接/关联到哪个事件
您可以使用@Relation简化提取相关数据的过程
请考虑以下内容：
事件实体（用于演示的简洁明了）
- 照片的父列将是\u id列
- 第二个被忽略（即被房间忽略）构造函数，否则房间会发出类似*警告的警告：有多个好的构造函数，房间会选择无参数构造函数*
稍有变化的照片实体：-
@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;
}
}
数据库（使用数据库检查器查看）显示：-
事件表：-
照片表格：-
@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;
}
}
public class EventWithPhotos {
@Embedded
Event event;
@Relation(entity = Photo.class,parentColumn = "_id",entityColumn = "id_event_ft")
List<Photo> photos;
}
@Dao
interface AllDao {
@Insert
long insert(Event event);
@Insert
long insert(Photo photo);
@Transaction
@Query("SELECT * FROM event")
List<EventWithPhotos> getAllEventsWithPhotos();
}
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());
}
}
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