perf(Admin::AttachmentsController): Preload deletable ids

Rendering the attachment index used to call Attachment#deletable? per
row to decide which delete buttons to enable. Each call issued up to
two queries (the polymorphic related_ingredients check and the LIKE
scan for ingredient-value references), so a page of N attachments cost
up to 2N extra queries in addition to the main fetch.

The controller now runs the deletable scope once for the ids on the
current page and stores the result in a Set. Because the scope relation
is chained as the subquery source of an IN clause, Rails applies the
same Ransack filter, sort, and pagination on the database side without
materializing the ids Ruby-side, keeping the check tight to the rows
the view actually renders.

Author: tvdeyen

app/controllers/alchemy/admin/attachments_controller.rb

@@ -39,6 +39,12 @@ def index
           .page(params[:page] || 1)
           .per(items_per_page)
 
+        # Preload deletable ids for the current page in a single query so the
+        # view can decide which delete buttons to enable without calling
+        # +deletable?+ (a two-query check) per row.
+        @deletable_attachment_ids =
+          Attachment.where(id: @attachments).deletable.pluck(:id).to_set
+
         if in_overlay?
           archive_overlay
         end

app/views/alchemy/admin/attachments/_files_list.html.erb

@@ -61,7 +61,7 @@
         file_attribute: 'file' %>
     <% end %>
     <% table.with_action(:destroy) do |attachment| %>
-      <% if attachment.deletable? %>
+      <% if @deletable_attachment_ids.include?(attachment.id) %>
         <sl-tooltip content="<%= Alchemy.t(:delete_file) %>">
           <%= link_to_confirm_dialog render_icon(:minus),
             Alchemy.t(:confirm_to_delete_file),

spec/controllers/alchemy/admin/attachments_controller_spec.rb

@@ -70,6 +70,32 @@ module Alchemy
         end
       end
 
+      describe "@deletable_attachment_ids" do
+        let!(:deletable_attachment) { create(:alchemy_attachment) }
+        let!(:referenced_attachment) { create(:alchemy_attachment) }
+
+        before do
+          create(
+            :alchemy_ingredient_richtext,
+            value: %(<a href="/attachment/#{referenced_attachment.id}/download">x</a>)
+          )
+        end
+
+        it "is a Set of ids of attachments on the current page that are deletable" do
+          get :index
+          expect(assigns(:deletable_attachment_ids)).to be_a(Set)
+          expect(assigns(:deletable_attachment_ids)).to include(deletable_attachment.id)
+          expect(assigns(:deletable_attachment_ids)).not_to include(referenced_attachment.id)
+        end
+
+        it "is scoped to the current page only" do
+          get :index, params: {per_page: 1}
+          # With per_page: 1 only one attachment is on the current page,
+          # so the set must have at most one element.
+          expect(assigns(:deletable_attachment_ids).size).to be <= 1
+        end
+      end
+
       describe "by_file_type filter" do
         let!(:png) { create(:alchemy_attachment) }