Multiselect based on Featured Category select field

I have a series of categories and my site settings contains a “featuredcategory” select field. A user can choose a category from this (for example Photography). After that I have a multiselect field and I would like to only show pages that exist within the previously selected category. So I choose Photography in the select, the multiselect shows the post tagged with Photography and then I can make a selection from there.

If I write a specific category into the filterBy query (eg “categories/photography”) it shows the category posts correctly but ideally, I would like the select field to control whats presented under the mutliselect. Below is my blueprint but perhaps I’m missing something or this isn’t possible, are nested queries like this doable? Any help much appreciated.


  label: Featured Category
  type: select
  options: query
     fetch: site.find("categories").children
     text: "{{ page.title }}"
     value: "{{ page.slug }}"

  label: Featured Category Selection
  type: multiselect
  options: query
     fetch: site.index.filterBy("categories", site.featuredcategory)

This kind of filtering is not possible, at least not as long as the value of the first field is not saved to file. But what you really want is the second field to react on changes in the first field. To achieve this, you would need a custom field though, that listens to changes in the first field. So, not possible out of the box, I’m afraid.