medium_preload_functions.ex

def category_preloader(category_names) when is_list(category_names) do
  # category_names are already deduplicated
  Enum.map(category_names, &RemoteService.retrieve_category_info/1)
end

query =
  from(p in Post,
    where: p.id in ^post_ids,
    preload: [category: ^&category_preloader/1]
  )

posts = Repo.all(query)
refute Enum.any?(posts, &(&1.category == nil))

# alternatively, concurrent version
def concurrent_category_preloader(category_names) when is_list(category_names) do
  category_names
  |> Enum.map(fn name -> Task.async(RemoteService, :retrieve_category_info, [name]) end)
  |> Enum.map(fn task -> Task.await(task) end)
end