Android Room：如何建立关系模型？

Question

我刚刚开始使用 Room，虽然一切看起来都非常直观，但我目前并不真正理解我如何处理关系。

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)

您应该如何为多对多或一对多关系建模？
这在实践中会是什么样子（例如 DAO + 实体）？

Answer 1

您可以使用 @Relation 注释来处理 Room 中的关系。

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的文档中有令人困惑的示例。我已经在我的另一个答案中写下了步骤和一些基本解释。您可以)

Answer 2

我创建了一个简单的便捷方法，可以手动填充一对多关系。因此，例如，如果您在 Country 和 City 之间存在一对多关系，则可以使用该方法在 Country.

中手动填充 cityList 属性

/**
 * @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);
    }
}

这就是我的使用方式。

   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;
    });

Android Room：如何建立关系模型？

Android Room: How to model relationships?

android

database

sqlite

orm

android-room