yii\mongodb\ActiveQuery::getCollection PHP Method

    public function getCollection($db = null)
    {
        /* @var $modelClass ActiveRecord */
        $modelClass = $this->modelClass;
        if ($db === null) {
            $db = $modelClass::getDb();
        }
        if ($this->from === null) {
            $this->from = $modelClass::collectionName();
        }
        return $db->getCollection($this->from);
    }