Yii2 Select only few columns from related model

Your code should go this way.

public function actionGetItems()
{
    $items = \app\models\WarehouseItems::find()
        ->joinWith([
             /*
              *You need to use alias and then must select index key from parent table
              *and foreign key from child table else your query will give an error as
              *undefined index **relation_key**
              */
            'user as u' => function($query){
                $query->select(['u.user_id', 'u.col1', 'u.col2']);
            }
        ])
        ->asArray()
        ->all();

    return $items;
}

You should simply modify the relation query like this :

$items = \app\models\WarehouseItems::find()->with([
    'user' => function ($query) {
        $query->select('id, col1, col2');
    }
])->asArray()->all();

Read more : http://www.yiiframework.com/doc-2.0/yii-db-activequerytrait.html#with()-detail