perf(Admin::PicturesController): Preload deletable ids

Rendering the picture archive used to call Picture#deletable? per row
to decide which delete buttons to show. 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 pictures 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. The same preload is wired to the update
action since it re-renders the picture partial via turbo stream.

Author: tvdeyen

app/controllers/alchemy/admin/pictures_controller.rb

@@ -8,6 +8,8 @@ class PicturesController < Alchemy::Admin::ResourcesController
       include CurrentLanguage
       include PictureDescriptionsFormHelper
 
+      before_action :load_pictures, only: :index
+
       before_action :load_resource,
         only: [:edit, :update, :url, :destroy]
 
@@ -19,6 +21,12 @@ class PicturesController < Alchemy::Admin::ResourcesController
         @picture = Picture.find(params[:id])
       end
 
+      # Preload deletable ids for the current page (index) or the current
+      # resource (update, which re-renders the +_picture+ partial via a
+      # turbo stream). One query replaces the per-row +deletable?+ check
+      # that would otherwise fire for every picture the view renders.
+      before_action :load_deletable_picture_ids, only: [:index, :update]
+
       add_alchemy_filter :by_file_format, type: :select, options: ->(query) do
         Alchemy::Picture.file_formats(query.result)
       end
@@ -30,8 +38,6 @@ class PicturesController < Alchemy::Admin::ResourcesController
       helper_method :picture_offset
 
       def index
-        @pictures = filtered_pictures(page: params[:page])
-
         if in_overlay?
           archive_overlay
         end
@@ -159,6 +165,18 @@ def items_per_page_options
 
       private
 
+      def load_pictures
+        @pictures = filtered_pictures(page: params[:page])
+      end
+
+      def load_deletable_picture_ids
+        @deletable_picture_ids = Picture
+          .where(id: @pictures || @picture)
+          .deletable
+          .pluck(:id)
+          .to_set
+      end
+
       def picture_offset
         ((params[:page] || 1).to_i - 1) * items_per_page
       end

app/views/alchemy/admin/pictures/_picture.html.erb

@@ -2,7 +2,7 @@
   <span class="picture_tool select">
     <%= check_box_tag "picture_ids[]", picture.id %>
   </span>
-  <% if picture.deletable? && can?(:destroy, picture) %>
+  <% if @deletable_picture_ids.include?(picture.id) && can?(:destroy, picture) %>
   <div class="picture_tool delete">
     <sl-tooltip content="<%= Alchemy.t('Delete image') %>">
       <%= link_to_confirm_dialog(

spec/controllers/alchemy/admin/pictures_controller_spec.rb

@@ -136,6 +136,27 @@ module Alchemy
           end
         end
       end
+
+      describe "@deletable_picture_ids" do
+        let!(:deletable_picture) { create(:alchemy_picture) }
+        let!(:referenced_picture) { create(:alchemy_picture) }
+
+        before do
+          create(:alchemy_ingredient_picture, related_object: referenced_picture)
+        end
+
+        it "is a Set of ids of pictures on the current page that are deletable" do
+          get :index
+          expect(assigns(:deletable_picture_ids)).to be_a(Set)
+          expect(assigns(:deletable_picture_ids)).to include(deletable_picture.id)
+          expect(assigns(:deletable_picture_ids)).not_to include(referenced_picture.id)
+        end
+
+        it "is scoped to the current page only" do
+          get :index, params: {per_page: 1}
+          expect(assigns(:deletable_picture_ids).size).to be <= 1
+        end
+      end
     end
 
     describe "#create" do