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

Document is really confusing. Try with just below classes:

1) User Entity:

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

2) Pet Entity:

@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 refers to Embedded User table's id column,
• entityColumn refers to Pet table's userId (User - Pet relation) column,
• entity refers to table(Pet) which has relation with User table.

4) UserDao Dao:

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

Now try loadUsersWithPets(), which returns the users with their list of pets.

Edit: See my other answer for many ot many relation.