Realm-java: Query recursive nested objects

[joaocruz04]

Hello,

I'm using Realm 3.1.0 for persistence, and I came across a problem regarding fetching recursive nested objects. Below is a Category class example:

open class Category : RealmObject() {
    @PrimaryKey
    open var id: Int = 0
    open var name:String? = ""
    open var child_categories: RealmList<Category>? = null
}

After generating the Category hierarchy I persist like this:

realm.beginTransaction()
realm.copyToRealm(categories)
realm.commitTransaction()

where "categories" is a list of the high-level categories, with all their child categories. This persist all data. The problem is querying Realm to return only the high-level categories, but if do a findAll(), it returns all categories regardless of level.
Do I need to model the Category class differently, adding a "level" field? I would like to avoid that..

Thanks in advance,
João

[cmelchior]

Hi @joaocruz04
If you do something like where(Category.class).findAll() it will find all categories no matter where they are. If you only want categories at the "bottom", you could do something like:

where(Category.class).isEmpty("child_categories").findAll()

If you have 3 levels it would be impossible to tell the difference between level 1 and 2, since they would both have data in their child_categories list, so in that case there is no other way than adding a depth field or something like that.

[Zhuinden]

well you COULD model it as

open class Category : RealmObject() {
    @PrimaryKey
    open var id: Int = 0
    open var name:String? = ""
    open var parent_category: Category? = null
    open var child_categories: RealmList<Category>? = null
}

and top level means realm.where(Category.class).isNull("parent_category"):

[joaocruz04]

So it seems I should change the model a bit. Thank you guys for the replies!

[Zhuinden]

@joaocruz04 I've had to do this before 😄

public class FeedCategory
        extends RealmObject {
    @PrimaryKey
    private long id;
    private int rank;
    private String title;
    @Index
    private String titleLowercase;
    private String color;
    private String subcategoryColor;
    private String icon;
    private RealmList<FeedCategory> subcategories;
    private FeedCategory parent;
 ```

and

``` java

public class FeedCategoryRepositoryImpl
        extends LongRealmRepositoryImpl<FeedCategory>
        implements FeedCategoryRepository {

    @Override
    public RealmResults<FeedCategory> findAllParentCategories(Realm realm) {
        return query(realm).isNull(FeedCategoryFields.PARENT).findAllSorted(FeedCategoryFields.RANK);
    }