public class UserModel extends RealmObject {
    @PrimaryKey
    public Long id;
    public String firstName;
    public String lastName;
    public UserSettings userSettingsModel;
}

public class UserSettingsModel extends RealmObject {
    @PrimaryKey
    private Long id;
    public String email;
    public RealmList<Car> cars;
}

public class Car extends RealmObject {
    @PrimaryKey
    private Long id;
    public String model;
}