Realm on Android - How to select multiple objects by list of ids (@PrimaryKey)?

Update:

Realm 1.2.0 has added RealmQuery.in() for a comparison against multiple values. The documentation details all the available overloads. This one is the method we can use if our ids are Integers:

public RealmQuery<E> in(String fieldName, Integer[] values)

Original answer:

The answer from @ChristianMelchior returns all articles if the list of ids is empty. I want it to return an empty RealmResults<Article>. That's what I've ended up doing:

Set<Integer> articleIds = this.getArticleIds();
RealmQuery<Article> query = realm.where(Article.class);
if (articleIds.size() == 0) {
    // We want to return an empty list if the list of ids is empty. 
    // Just use alwaysFalse
    query = query.alwaysFalse();
} else {
    int i = 0;
    for (int id : articleIds) {
        // The or() operator requires left hand and right hand elements. 
        // If articleIds had only one element then it would crash with
        // "Missing right-hand side of OR"
        if (i++ > 0) {
            query = query.or();
        }
        query = query.equalTo("id", id);
    }
}
return query.findAll();

This is the way Realm does it since 1.2.0:

public RealmQuery<E> in(String fieldName, String[] values) {
    if (values == null || values.length == 0) {
        throw new IllegalArgumentException(EMPTY_VALUES);
    }
    beginGroup().equalTo(fieldName, values[0]);
    for (int i = 1; i < values.length; i++) {
        or().equalTo(fieldName, values[i]);
    }
    return endGroup();
}

Previously this is how I did it

The Realm Java API's doesn't support this yet unfortunately. You can follow the feature request here https://github.com/realm/realm-java/issues/841

The current work-around would be to build up the query yourself in a for-loop:

RealmResults<Article> articles = realm.allObjects(Article.class);
RealmQuery q = articles.where();
for (int id : ids) {
    q = q.equalTo("id", id);
}
RealmResults<Article> filteredArticles = q.findAll();

Now realm v 1.2.0 support RealmQuery.in() for a comparison against multiple values.