/**
 * @return yiidbActiveQuery
 */
public function getComplexDocument()
{
    return $this->hasMany(ComplexDocument::className(), ['complex_id' => 'id']);
}

/**
 * @return yiidbActiveQuery
 */
public function getDocuments()
{
    return $this->hasMany(Documents::className(), ['id' => 'document_id'])
        ->via('complexDocument');
}

public function beforeDelete()
{
    foreach ($this->documents as $document){
        var_dump($document);
    }
    return parent::beforeDelete();
}