Android Room: How to model relationships?

Question

I have just started working with Room and although everything seems to be pretty intuitive I currently don't really understand how exactly I could handle relationships.

Because SQLite is a relational database, you can specify relationships between objects. Even though most ORM libraries allow entity objects to reference each other, Room explicitly forbids this. Even though you cannot use direct relationships, Room still allows you to define Foreign Key constraints between entities.(Source: https://developer.android.com/topic/libraries/architecture/room.html#no-object-references)

1. How should you model a Many to Many or One to Many Relationship?
2. What would this look like in practice (example DAOs + Entities)?

Answer 1

You can use @Relation annotation to handle relations at Room.

A convenience annotation which can be used in a Pojo to automatically fetch relation entities. When the Pojo is returned from a query, all of its relations are also fetched by Room.

See document.

(Google's document has confusing examples. I have written the steps and some basic explanation at my another answer. You can check it out)

Answer 2

I created a simple Convenience Method that populates manually a one to many relationship. So for example if you have a one to many between Country and City , you can use the method to manually populate the cityList property in Country.

/**
 * @param tableOne The table that contains the PK. We are not using annotations right now so the pk should be exposed via a getter getId();
 * @param tableTwo The table that contains the FK. We are not using annotations right now so the Fk should be exposed via a getter get{TableOneName}Id(); eg. getCountryId();
 * @param <T1>     Table One Type
 * @param <T2>     Table Two Type
 * @throws NoSuchFieldException
 * @throws IllegalAccessException
 * @throws NoSuchMethodException
 * @throws InvocationTargetException
 */
private static <T1, T2> void oneToMany(List<T1> tableOne, List<T2> tableTwo) throws NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {

    String tableOneName = tableOne.get(0).getClass().getSimpleName();
    String tableTwoName = tableTwo.get(0).getClass().getSimpleName();
    for (T1 t1 :
            tableOne) {
        Method method = t1.getClass().getMethod("getId");
        Integer pkId = (Integer) method.invoke(t1);
        List<T2> listForCurrentId = new ArrayList<>();
        for (T2 t2 : tableTwo) {
            Method fkMethod = t2.getClass().getDeclaredMethod("get".concat(tableOneName).concat("Id"));
            Integer fkId = (Integer) fkMethod.invoke(t2);
            if (pkId == fkId) {
                listForCurrentId.add(t2);
            }
        }
        Method tableTwoList = t1.getClass().getMethod("set".concat(tableTwoName).concat("List"), List.class);
        tableTwoList.invoke(t1, listForCurrentId);
    }
}

This is how I use it .

   SystemDefaults systemDefaults = new SystemDefaults();
    return Single.zip(systemDao.getRoles(), systemDao.getCountries(), systemDao.getCities(), (roles, countries, cities) -> {
        systemDefaults.setRoles(roles);
        *ConvenienceMethods.oneToMany(countries,cities);*
        systemDefaults.setCountries(countries);
        return systemDefaults;
    });