Define Unique Fields in Domain Model with Agile Data

Controller_UniqueFields.php

<?php

namespace smbo;

class Controller_UniqueFields {
    use \atk4\core\InitializerTrait {
        init as _init;
    }
    use \atk4\core\TrackableTrait;
    use \atk4\core\AppScopeTrait;

    protected $fields = null;

    function init() {
        $this->_init();

        // by default make 'name' unique
        if (!$this->fields) {
            $this->fields = [$this->owner->title_field];
        }

        $this->owner->addHook('beforeSave', $this);
    }

    function beforeSave($m) 
    {
        foreach ($this->fields as $field) {
            if ($m->dirty[$field]) {
                $mm = clone $m;
                $mm->tryLoadBy($field, $m[$field]);

                if ($mm->loaded()) {
                    throw new \atk4\core\Exception(['Duplicate record exists', 'field'=>$field, 'value'=>$m[$field]]);
                }
            }
        }
    }
}