In the simple cases, you can use the RunsOnRelation trait to run any existing filter on a relation.

However, if you have more complex requirements, like nested relations, you can manually use Laravel's whereHas method or use the RunsOnRelation trait with a dot-notated relation name.

#Basic Relation Filter

For most cases, just using the trait is enough.

class PostTitleFilter extends StringFilter
{
    use RunsOnRelation;

    protected string $relation = 'post';
    protected string $field = 'title';
}

#Nested Relations

You can also filter through nested relations by using a dot-notated relation name:

class AuthorPostTitleFilter extends StringFilter
{
    use RunsOnRelation;

    protected string $relation = 'author.posts';
    protected string $field = 'title';
}

This will automatically wrap the logic in whereHas('author.posts', ...) and correctly qualify the title column using the posts table name.

#Complex Custom Relation Filters

If you need more control, such as adding additional constraints to the whereHas query, you can manually override the applyFilter method while still using the trait:

class CustomRelationFilter extends StringFilter
{
    use RunsOnRelation;

    protected string $relation = 'posts';
    protected string $field = 'title';

    public function applyFilter(Builder $query): Builder
    {
        // Add custom constraints inside the whereHas closure
        $query->where('published', true);

        // Call parent logic to apply the string filter on the title field
        return parent::applyFilter($query);
    }
}