Android Persistence room：“无法弄清楚如何从光标读取此字段”

Question

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在https://developer.android.com/reference/android/arch/persistence/room/Relation.html 找到的示例：

 @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 {
     @Query("SELECT * from User")
     public List<UserAllPets> loadUserAndPets();
 }

我收到以下错误

    ...error: Cannot figure out how to read this field from a cursor.

关于：

 private java.util.List<?> pets;

我想指出，我发现他们文档中的某些内容确实令人困惑。例如缺少@PrimaryKey 以及User 类缺少@Entity 注释的事实，尽管它应该是一个实体（据我所知）。有人遇到同样的问题吗？提前非常感谢

Answer 1

Document 真是令人困惑。试试下面的类：

1) 用户实体：

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

2) 宠物实体：

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

3) UserWithPets POJO：

// Note: No annotation required at this class definition.
public class UserWithPets {
   @Embedded
   public User user;

   @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class)
   public List<Pet> pets; // or use simply 'List pets;'


   /* Alternatively you can use projection to fetch a specific column (i.e. only name of the pets) from related Pet table. You can uncomment and try below;

   @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class, projection = "name")
   public List<String> pets; 
   */
}

• parentColumn 指的是嵌入式User 表的id 列，
• entityColumn 指的是Pet 表的userId (User - Pet 关系) 列，
• entity 指的是与User 表有关系的表(Pet)。

4) UserDao 道：

@Dao
public interface UserDao {
    @Query("SELECT * FROM User")
    public List<UserWithPets> loadUsersWithPets();
}

现在试试loadUsersWithPets()，它会返回用户及其宠物列表。

编辑：请参阅我的other answer 了解许多关系。