Sort pages throughout different date types

1

In a blog page I created, the user can create posts labeled either as “news” or “event”. The “event”-posts have an additional field for an event date.

post.yml

fields:
  category:
    type: select
    required: true
    default: news
    options:
      news: News
      event: Event
  eventdate:
    label: Event date
    type: date
    display: DD-MM-YYYY
    when: 
      category: event
    required: true
  text:
    type: textarea

In the template, the posts are sorted by publication date:

blog.php

<ul> 
  <?php foreach ($page->children()->listed()->sortBy('date', 'desc') as $post) : ?>
     ...
  <?php endforeach ?>
</ul>

Now here comes my problem: If a new event gets added spontaneously, it always appears at the top of the sorting, even if the event date is earlier than that of a previously created event (because the sorting is done by publication date).

If I set the sorting to

<ul> 
  <?php foreach ($page->children()->listed()->sortBy('eventdate', 'desc', 'date', 'desc') as $post) : ?>
     ...
  <?php endforeach ?>
</ul>

all the events are rendered first (because there is no event date on the “news” categorized posts).

So how can I fix that?
I would like to use the event date for the event posts and the publication date for the news posts and then sort all posts by their date.

Any help?

2

What is the publication date?

3

I’m sorry I forgot to mention that there is a date field for the news posts:

post.yml

fields:
  ...
  date:
    label: Publishing date
    type: date
    display: DD-MM-YYYY
    default: today
    when: 
      category: news
4

Then you can create either a page model method (date or eventdate) for one of the page types or use sortBy with a callback depending on template, see Sorting collections | Kirby CMS

5

A page model method returning the right date for each category helped. Thank you!