Android Persistence room : "Cannot figure out how to read this field from a cursor"

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 https://developer.android.com/reference/android/arch/persistence/room/Relation.html 中的示例:

 @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 注释这一事实，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真的很困惑。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 指嵌入式User 表的id 列，
• entityColumn 引用Pet 表的userId(User - Pet 关系)列，
• entity 是指与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:见我的other answer对于很多很多关系。