Once you have defined a filter, you are ready to attach it to a resource. Each resource generated by Nova contains a filters
method. To attach a filter to a resource, you should simply add it to the array of filters returned by this method:
/**
* Get the filters available for the resource.
*
* @param \Laravel\Nova\Http\Requests\NovaRequest $request
* @return array
*/
public function filters(NovaRequest $request)
{
return [
new Filters\UserType,
];
}