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

Question

I'm trying to create a relation between two database tables using the new Android Persistence Room Library. I looked at the documentation and tried to implement the example found at https://developer.android.com/reference/android/arch/persistence/room/Relation.html:

 @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();
 }

I get the following error

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

in relation to:

 private java.util.List<?> pets;

I would like to point out that I found some things in their docs really confusing. For example the lack of @PrimaryKey and also the fact that the User class is missing the @Entity annotation, although it's supposed to be an entity (as fas as I see it). Did anybody run into the same problem? Thanks a lot in advance

Answer 1

文档真的很混乱。尝试以下课程：

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指Pet表的userId（User-Pet关系）列，
• entity指与tablePet有关系的table() User。

4）用户道道：

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

现在尝试loadUsersWithPets()，它返回用户及其宠物列表。

编辑：有关许多关系，请参阅我的其他答案。