Android 持久空间："Cannot figure out how to read this field from a cursor"

Question

我正在尝试使用新的 Android Persistence Room 库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 中找到的示例：

 @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;

我想指出的是，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 class 缺少 @Entity 注释的事实，尽管它应该是一个实体（正如我所看到的那样).有人运行遇到同样的问题吗？非常感谢

Answer 1

Document 真是令人费解。试试下面类:

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 table 的 id 列，
entityColumn 指的是 Pet table 的 userId（User - Pet 关系）列，
entity 指的是 table(Pet) 与 User table.

4) UserDao道:

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

现在尝试 loadUsersWithPets()，其中 returns 用户及其宠物列表。

编辑： 请参阅我的 other answer 了解许多关系。

Android 持久空间："Cannot figure out how to read this field from a cursor"

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

sqlite

android

android-room

android-architecture-components