Android Room: Insert relation entities using Room

You can do this by changing your Dao from an interface to an abstract class.

@Dao
public abstract class UserDao {

    public void insertPetsForUser(User user, List<Pet> pets){

        for(Pet pet : pets){
            pet.setUserId(user.getId());
        }

        _insertAll(pets);
    }

    @Insert
    abstract void _insertAll(List<Pet> pets);  //this could go in a PetDao instead...

    @Insert
    public abstract void insertUser(User user);

    @Query("SELECT * FROM User")
    abstract List<UserWithPets> loadUsersWithPets();
}

You can also go further by having a User object have an @Ignored List<Pet> pets

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

    @Ignored
    public List<Pet> pets
}

and then the Dao can map UserWithPets to User:

public List<User> getUsers() {
    List<UserWithPets> usersWithPets = loadUserWithPets();
    List<User> users = new ArrayList<User>(usersWithPets.size())
    for(UserWithPets userWithPets: usersWithPets) {
        userWithPets.user.pets = userWithPets.pets;
        users.add(userWithPets.user);
    }
    return users;
}

This leaves you with the full Dao:

@Dao
public abstract class UserDao {

    public void insertAll(List<User> users) {
        for(User user:users) {
           if(user.pets != null) {
               insertPetsForUser(user, user.pets);
           }
        }
        _insertAll(users);
    }

    private void insertPetsForUser(User user, List<Pet> pets){

        for(Pet pet : pets){
            pet.setUserId(user.getId());
        }

        _insertAll(pets);
    }

    public List<User> getUsersWithPetsEagerlyLoaded() {
        List<UserWithPets> usersWithPets = _loadUsersWithPets();
        List<User> users = new ArrayList<User>(usersWithPets.size())
        for(UserWithPets userWithPets: usersWithPets) {
            userWithPets.user.pets = userWithPets.pets;
            users.add(userWithPets.user);
        }
        return users;
    }


    //package private methods so that wrapper methods are used, Room allows for this, but not private methods, hence the underscores to put people off using them :)
    @Insert
    abstract void _insertAll(List<Pet> pets);

    @Insert
    abstract void _insertAll(List<User> users);

    @Query("SELECT * FROM User")
    abstract List<UserWithPets> _loadUsersWithPets();
}

You may want to have the insertAll(List<Pet>) and insertPetsForUser(User, List<Pet>) methods in a PetDAO instead... how you partition your DAOs is up to you! :)

Anyway, it's just another option. Wrapping your DAOs in DataSource objects also works.

There is no native solution till any update in Room Library but you can do this by a trick. Find below mentioned.

1. Just Create a User with Pets (Ignore pets). Add getter and setter. Notice that we have to set our Id's manually later and can't use autogenerate.

   @Entity
   public class User {
         @PrimaryKey
         public int id; 
   
         @Ignore
         private List<Pet> petList;
   }
   

2. Create a Pet.

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

3. The UserDao should be an abstract class instead of an Interface. Then finally in your UserDao.

   @Insert
   public abstract void insertUser(User user);
   
   @Insert
   public abstract void insertPetList(List<Pet> pets);
   
   @Query("SELECT * FROM User WHERE id =:id")
   public abstract User getUser(int id);
   
   @Query("SELECT * FROM Pet WHERE userId =:userId")
   public abstract List<Pet> getPetList(int userId);
   
   public void insertUserWithPet(User user) {
       List<Pet> pets = user.getPetList();
       for (int i = 0; i < pets.size(); i++) {
           pets.get(i).setUserId(user.getId());
       }
       insertPetList(pets);
       insertUser(user);
   }
   
   public User getUserWithPets(int id) {
       User user = getUser(id);
       List<Pet> pets = getPetList(id);
       user.setPetList(pets);
       return user;
   }
   

Your problem can be solved by this without creating UserWithPets POJO.

As Room does not manage the Relations of the entities, you have to set the userId on each pet yourself and save them. As long as there are not too many pets at once, I'd use an insertAll method to keep it short.

@Dao
public interface PetDao {
    @Insert
    void insertAll(List<Pet> pets);
}

I don't think there's any better way at the moment.

To make the handling easier, I'd use an abstraction in the layer above the DAOs:

public void insertPetsForUser(User user, List<Pet> pets){

    for(Pet pet : pets){
        pet.setUserId(user.getId());
    }

    petDao.insertAll(pets);
}