如何在android机房中表示嵌套的一对多关系？

Question

Here are sample POJO:

@Entity
public class User {
  @PrimaryKey
  public String id;
}

@Entity
public class Pet {
  @PrimaryKey
  public String id;
  public String userId;
  public String petName;
}

@Entity
public class Category {
  @PrimaryKey
  public String id;
  public String petId;
  public String categoryName;
}

Here is my UsersWithPets class to fetch all the users along with pets.

public class UserWithPets {
  @Embedded
  public User user;
  @Relation(parentColumn = "id", entityColumn = "userId", entity =   Pet.class)
  public List<Pet> pets;
}

My user DAO

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

and userdb

@Database(entities = {User.class, Pet.class, Category.class}, version = 1)
@TypeConverters(DateConverter.class)
public abstract class UsersDb extends RoomDatabase {

  private static UsersDb INSTANCE;

  public abstract UserDao userDao();

  public static UsersDb getInstance(Context context) {
      if (INSTANCE == null) {
          synchronized (UsersDb.class) {
              if (INSTANCE == null) {
                  INSTANCE = Room.databaseBuilder(context.getApplicationContext(), UsersDb.class,
                          "User.db")
                          .build();
              }
          }
      }
      return INSTANCE;
  }
}

The "UserWithPets" embeds user object and relates with its associated list of "Pet", but how do I fetch list of "Category" if my "Pet" POJO can have 1-many relationship with "Category". Also the user DAO only returns all the "User" along with its "List" by mapping the id between user.id and pet.userId, if my "Pet" POJO can have many "Category"(List of Category), how do I create my DAO and abstract database class such that querying for a particular "User" id will return me the "User" object along with its "List of Pets" and individual "Pet" containing a "List of Category" and querying for all will return all the "List of Users" wherein each User contains a "List of Pet" and each Pet contains a "List of Category"

Answer 1

You need to map your query to a POJO/Java Bean class like PetWithCategories:

public class UserWithPets {
  @Embedded
  public User user;
  @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class)
  public List<PetWithCategories> pets;
}

public class PetWithCategories {
  @Embedded
  public Pet pet;
  @Relation(parentColumn = "id", entityColumn = "petId")
  public List<Category> categories;
}

Answer 2

Probably it would be easier to start additional database requests after you finished the first one. I don't see any way to realize that via @Relation or with inner joins as this is not a single value you want to return. You could use left join, but that would give you multiple pets, only with different category objects.

List<UserWithPets> list = userDao.loadUsersWithPets();
for(UserWithPets user : list) {
   for(Pet pet : user.pets) {
      pet.categories = categorieDao.getCategoriesByPetId(pet.id); 
      //pet.categories should have the @Ignore annotation
   }
}

...

@Dao
public interface CategoryDao {

   @Query("SELECT * FROM Category WHERE petId = :petId")
   List<Categories> getCategoriesByPetId(int petId);
}