相关疑难解决方法(0)

我正在尝试使用新的Android Persistence Room Library在两个数据库表之间创建关系.我查看了文档,并尝试实现https://developer.android.com/reference/android/arch/persistence/room/Relation.html上的示例:

 @Entity
 public class User {
 @PrimaryKey
     int id;
 }

 @Entity
 public class Pet {
     @PrimaryKey
     int id;
     int userId;
     String name;

 }

 @Dao
 public interface UserDao {
     @Query("SELECT * from User")
     public List<User> loadUser();
 }

 @Dao
 public interface PetDao {
     @Query("SELECT * from Pet")
     public List<Pet> loadUserAndPets();
 }


 public class UserAllPets {
     @Embedded
     public User user;
     @Relation(parentColumn = "user.id", entityColumn = "userId", entity = Pet.class)
     public List pets;
 }

 @Dao
 public interface UserPetDao { …

我正在开发一个 android 应用程序，并在 Android 操作系统中使用新的架构组件：LiveData、ViewModel 和 Room。我对 Room 实现有一个关于创建 @Relation 的小问题，它返回 JOIN 查询（​​多对多关系）的结果。

我的数据库结构如下所示：

@Entity
public class Student{
  @PrimaryKey
  private int id;
  private String name;
  private String email;
} 

@Entity
public class Group{
  @PrimaryKey
  private int id;
  private String name;
}

@Entity(foreignKeys = {
            @ForeignKey(entity = Student.class,
                    parentColumns = "id",
                    childColumns = "student_id"),
            @ForeignKey(entity = Group.class,
                    parentColumns = "id",
                    childColumns = "group_id")
    })
public class StudentGroup{

  private int studentId;
  private int groupId;
}

我如何才能为特定学生获取所有组，像这样？

public class StudentWithGroups{
  @Relation(parentColumn = "id", …