How to implement a many to many relations with Android Room Persistence Library?

Question

How to implement a many to many relations with Android Room Persistence Library?

One user may have one or many devices & One device may be owned by one or many users.

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

@Dao
public interface UserDao {
  @Query("select * from user") List<User> getAllUsers();

  @Query("select * from user where id = :id")
  User getUserById(long id);
}

@Entity
public class Device {

   public @PrimaryKey Long id;
   public String name;

}


@Dao
public interface DeviceDao {

    @Query("select * from device")
    List<Device> getAllDevices();
}


@Entity
public class UserDevice {
    public String userId;
    public String deviceId;
}

@Dao
public interface UserDeviceDao {

// List all devices by userId
// List all users by deviceId

}

Answer 1

Assuming a typo in UserDevice so id's are Long instead of String:

@Entity(primaryKeys = {"userId", "deviceId"})
public class UserDevice {
    public Long userId;
    public Long deviceId;
}    

You could try:

@Dao
public interface UserDeviceDao {

    // List all devices by userId
    @Query("select * from device " +
           " inner join userdevice on device.id = userdevice.deviceId " +
           " where userdevice.userId = :userId")
    List<Device> getUserDevices(Long userId);

    // List all users by deviceId
    @Query("select * from user " +
           " inner join userdevice on user.id = userdevice.userId " +
           " where userdevice.deviceId = :deviceId")
    List<User> getDeviceUsers(Long deviceId);
}