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Overview

There is an convenience method on Filter::relation that allows you to easily define nested relationships. 
public function allowedFilters(): AllowedFilterList
{
    return Filter::only(
        Filter::relation('posts', [FilterType::HAS])->includeRelationFields()
            ->andNestedRelation(
                Filter::relation('comments', [FilterType::HAS])->includeRelationFields()
            ),
    );
}