Running the can_ functions on a domain triggers N queries if used in a list

[StephanH90]

Describe the bug
When you use the can_:action_name? functions that get generated on a domain in a list, it creates 1 query for every row.

To Reproduce
Let's assume we have a policy like this:

policy action :delete do
    authorize_if relates_to_actor_via(:users)
    # I have also tried other flavours such as:
    # authorize_if expr(exists(users, id == ^actor(:id))) # <- also produces a query for every row
end

A common use-case would be to only show the delete button if this authorization passes. However, if you use the function MyDomain.can_delete?(some_resource, user) you actually get 1 query for every resource.

Expected behavior
I would have assumed that Ash tries to use the preloaded relationship if possible.

[zachdaniel]

You get 1 query for every record? That shouldn't be the case.

[zachdaniel]

I think? I'll have to double check that implementation. Could you provide a more concrete example of how you're calling it?

[StephanH90]

@zachdaniel I prepared a little MR that illustrates the problem: https://github.com/ash-project/ash/pull/1661/files

Otherwise, here is the main bit of my change:

# Post resource
defmodule Post do
  # stuff

  actions do
    # stuff

    update :update_title do
      accept [:title]
    end
  end

  relationships do
    belongs_to :user, User, public?: true
  end

  policies do
    policy action :update_title do
      authorize_if relates_to_actor_via(:user)
    end
  end
end

test "a can check doesn't rerun queries" do
  start_supervised({Ash.Test.Authorizer, {}})

  user =
    User
    |> Ash.Changeset.for_create(:create)
    |> Ash.create!(authorize?: false)

  post =
    Post
    |> Ash.Changeset.for_create(:create, %{title: "test", user_id: user.id})
    |> Ash.create!(authorize?: false)
    |> Ash.load!(:user)

  # ! This causes an extra SQL query (!not when running against ETS)
  Domain.can_update_post_title(user, post)

  # In my actual usecase I have something like this in my liveview:
  # <div :for={item <- @items}>
  #   <button :if={MyDomain.can_delete(@current_user, item)}>Delete</button>
  #   {item.name}
  # </div>

  # This causes 1 query for every row.
end

If you check out my comment at the end of the test, I show an example from my actual use case. In my live view I want to show a delete button for every item. However, the user can only delete their own item.