Is there a way to return an empty DataList on purpose?

Questions
1

Silverstripe Version: 4 or 3, this subject is not about a specific version. If a solution can be found in either version, I would be glad.

Question:

Sometimes when doing filtering on DataLists, there are exceptional cases where I would like to make the DataList empty on purpose.

This is what I have used so far, but it’s not efficient as it performs an unnecessary database query:

$empty_data_list = DataList::create('MyDataObject')->where('FALSE');

For example, I might want to show a list of DataObjects to logged in users only, but if a user is not logged in, she should see nothing in the list. I know this could be handled in a different way by checking the login status in the template that is used to output the list, but I would still like to know a general solution to this subject.

Thanks! :slight_smile:

2

I’ve encountered this problem before too, in situations where you want to be consistent and predictable with what you’re returning. I did a similar thing to you, from memory I think I did ->filter('ID', 0). If there’s a more efficient approach I’m interested to hear as well.

3

If you just need an empty list and nothing else, and you don’t want to refine that list, why not simply return an empty ArrayList? Both implement the SS_List interface and are quite interchangeable e.g. in templates.

4

I’ve considered this too, but like @JonoM, I would like a method’s return type to be as predictable as possible. ArrayList and DataList share a lot of common functions but there still is a possibility to end up using a method on the returned (empty) list that is only available if the list is a DataList. And if it happens to be an ArrayList instead of a DataList, it will crash the application.

I’m just trying to avoid possible errors in edge cases. And I also like to keep method return types and descriptions as simple as possible: no need to describe why a method usually returns a DataList and sometimes an ArrayList. :slight_smile: