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
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.