Sorting pages by two fields, sort only by 2nd field, when 1st field is empty

[intrigus]

Hello, I'd like to sort pages by two selectors.

I have a custom field "startdesprojekts" it is of the type date.

I want to sort the pages by "startdesprojekts". Since it's possible that there is no date defined I'd like to sort by creation time, if "startdesprojekts" is undefined.

My code looks like this, but it doesn't do what I want.

$items = $page->children("include=hidden, sort=-modified")->append($pages->find("inNewsAnzeigen=1"))->filter("sort=-startdesprojekts, sort=-created");

[LostKobrakai]

The sort selector does only fallback to the second parameter for items, where the first sort parameter is the same value. It's not combining both field values to sort afterwards – they might even be not comparable because of different types. 

[intrigus]

In my case, both fields are of the same type, both are dates.

Are you sure that there is no possibility to sort by field1 if it exists and sort by field2 if field1 doesn't exist?

So I'd like to first sort by field1 and only sort by field2 if fields1 doesn't exist.

[LostKobrakai]

That's a fact that nobody can rely on, therefore it's not present in the processwire selector api. You could however do in with raw mysql or in memory with php. 

[hheyne]

What about 2 PageArrays? Sort them depending of existence of the fields and then you can concatenate them afterwards.

[LostKobrakai]

Two page arrays result in this:

date 1 | date 2

-    1

-    2

-    3

1

2

3

4

Or the other way around. It's not merging the dates. But if all pages are fetched no matter what (no pagination) it works like that:

foreach($myPages as $p){
  $p->set('sortDate', $p->date1 ?: $p->date2); // Runtime field
}
$myPages->sort("sortDate");

[Macrura]

you would have 2 options i think (besides the runtime sort field as described above), one is to sort the Pagearray after , using ->sort(field1|field2) or you could store the sortdate as a hidden field on the page using a module to write that field everytime the page is saved