Support for ordering on non-indexed properties

[zerolab]

e.g. https://wagtailcms.slack.com/archives/C81FGJR2S/p1762869979474629

Using the image listing, if you search by something and try to order by the usage count you get:

Cannot sort search results with field "usage_count". Please add index.FilterField("usage_count") to CustomImage.search_fields.

However, usage_count is an annotation on the results queryset, so adding index.FilterField("usage_count") fails with images.CustomImage: (modelsearch.W004) CustomImage.search_fields contains non-existent field 'usage_count', as expected. The workaround in this case is to define

def usage_count(self):
    return self.get_usage().count()

on the CustomImage object, but IMHO, this should just work.

Note: this is not limited to just core images/docs

[laymonage]

I assume for ordering to work on search results, you need to also have the ordering field indexed on the search backend. I'm not familiar with how it works, but I think once you do a search(), operations are handed to the search backend. At which point it's no longer possible to order by something that only exists on the Django ORM level and not on the search backend (at least on non-database backends).

Annotations happen at query-time, and if the value wasn't indexed ahead of time, how would the search backend know how to order by the annotation?

If the above is correct, then Wagtail should define the FilterFields for those annotations by default, I think. Though my main concern would be whether that would incur any overhead that's too heavy to happen whenever a reindexing is triggered.

[zerolab]

This depends on the backend. e.g. the db backend will likely be fine with the annotations, and other more complex incantations. Looking a bit more, this is somewhat related to #31 too.

I think your assessment of the order of operations is correct. So the question is what can we sensibly add out of the box without massive overheads, and what should be a documentation task (e.g. your model / ModelViewSet has this custom column that you want to search/sort on)