Example of how to do searchable_fields with a many_many relation

Project.php

<?php

use SilverStripe\ORM\DataObject;
use SilverStripe\Forms\DropdownField;
use SilverStripe\ORM\Filters\ExactMatchFilter;

class Project extends DataObject
{
    private static $db = [
        'Title' => 'Varchar(200)',
    ];
  
    private static $many_many = [
        'Regions' => Region::class,
    ];
  
    private static $summary_fields = [
        'ID' => 'ID',
        'Title' => 'Name'
    ];

    private static $searchable_fields = [
        'Title',
        'Regions.ID'
    ];

    private static $default_sort = 'Title ASC';


    public function searchableFields()
    {
        $fields = parent::searchableFields();

        $fields['Regions.ID'] = [
            'filter' => ExactMatchFilter::class,
            'title' => 'Region',
            'field' => DropdownField::create('Region')->setSource(
                Region::get()
            )->setEmptyString('All regions')
        ];

        return $fields;
    }
}