Relatively common setup:

I have a Thing which has_many SubThing. In the index view (showing many Things) I want to display a link or a text if there are any “sub things”. Otherwise not.

Of course I could just Repo.preload(:sub_things) but I don’t need them here so I wanna be light on the db/memory etc.

I know that I can write some query that returns me its own data structure telling me which Thing record has how many SubThings (or id any at all) and I could use that second data structure to display/not display them. I don’t have that query but I’m confident that I can write it with relative ease

I think this is a relatively common problem, so the question is: “Is there a better/standard way to do this?” (my googling came up empty)

If I could wish for something then my Thing struct would have an additional attribute like:

%Thing{
  sub_things_exist?: true/false
  # or / and
  sub_things_count: 5
}

And I could get there with Repo.preload(:sub_things_exist?) or something similar.

Just to be clear I don’t want to introduce a counter cache (yet) it’s fine if it’s still queried from the database dynamically but ideally in one big query (this is on an index page with a lot of elements).

Thanks and cheers as always!