common/modules/relation/relationQueryTrait.php

<?php
namespace common\modules\relation;
trait relationQueryTrait {
    static public $cache_tree = [];
    /** @var \yii\db\ActiveQuery $this */
    static $model;
    /*
     * @return \yii\db\ActiveQuery
     */
    private function getModel()
    {
        if (empty(self::$model)) {
            $class = $this->modelClass;
            self::$model = new $class;
        }
        return self::$model;
    }
    public function getRelations($relationKey) {
    }
}