Ash.Query.after_action/2 with relationships

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1

I’m trying to understand how Ash.Query.after_action/2 works and when relationships get loaded during the action lifecycle.

The problem: In my read action I query some stories and a couple of relationships. I want to use a preparation to format these stories + the related data for the frontend. However, no relationships are loaded in the Ash.Query.after_action/2 function even when I load them before.

Here is a minimal example:

# story.ex resource
read :list_public do

  prepare build(
            load: [:tags]
          )

  prepare Tinytale.Ash.Stories.Preparations.Frontend
  pagination(offset?: true)
end

And my preparation:

defmodule App.Stories.Preparations.Frontend do
  use Ash.Resource.Preparation
 
  def prepare(query, _opts, _context) do
    query
    |> Ash.Query.after_action(fn _query, stories ->
      dbg(stories)
      # do something with the stories & relationships
      end)
  end
end

When I inspect the stories I can see that the :tags relationship is not available. When are relationships actually loaded during the action lifecycle?

I’ve checked the docs and they mention that relationships get asynchronously loaded. When does Ash.Query.after_action/2 hook in exactly and is there an option to force loading related data?

2

Loaded information is filled in after the hooks are run. Can you instead make a calculation (not an expression calculation) that returns instances of the target resource?

calculate :things, {:array, Thing}, ListThings

defmodule ListThings do
  use Ash.Resource.Calculation

  def load(_, _, _) do
    [:things]
  end

  def calculate(records, _, _) do
     ...
  end
end
1 Like
3

Thanks for the clarification. Your approach works, but now I have another question. :grin:

Does the load/3 in this Calculation result in another database roundtrip or does Ash pick up relationships that are already loaded through the action’s preparations?

I’m asking because in my example I have omitted a custom preparation that loads the first story.scenes.illustrations to use as the ‘book_cover’ (the initial goal with Ash.Query.after_action/2 was to get an object with selected Story attributes + relationships for the frontend)

I’d rather avoid loading [scenes: :illustrations] and other relationships again. In that case I think piping the records through a plain Elixir function after the read action would be the better way.

4

The calculation engine combines loads and creates a minimal load statement to satisfy both the requested loads and requirements for calculations. If you have authorization policies on the resources in question and are running the query with authorization, though, it won’t combine the loads in this specific case, because the user loads are run with authorization and the calc dependency loads are not (intentionally, so you can define calculated properties however you want. I will add an option to authorize calc dependency loads just like regular loads in the future).

1 Like