Fix N+1 queries for ActiveStorage attachments

Closes #3282

lib/rails_admin/config/fields/types/active_storage.rb

@@ -22,6 +22,10 @@ class ActiveStorage < RailsAdmin::Config::Fields::Types::FileUpload
             end
           end
 
+          register_instance_option :eager_load do
+            {"#{name}_attachment": :blob}
+          end
+
           def resource_url(thumb = false)
             return nil unless value
             if thumb && value.variable?

lib/rails_admin/config/fields/types/multiple_active_storage.rb

@@ -43,6 +43,10 @@ def resource_url(thumb = false)
           register_instance_option :delete_method do
             "remove_#{name}" if bindings[:object].respond_to?("remove_#{name}")
           end
+
+          register_instance_option :eager_load do
+            {"#{name}_attachments": :blob}
+          end
         end
       end
     end

spec/rails_admin/config/fields/types/active_storage_spec.rb

@@ -84,4 +84,10 @@
       end
     end
   end
+
+  describe '#eager_load' do
+    it 'points to associations to be eager-loaded' do
+      expect(field.eager_load).to eq({active_storage_asset_attachment: :blob})
+    end
+  end
 end if defined?(ActiveStorage)

spec/rails_admin/config/fields/types/multiple_active_storage_spec.rb

@@ -88,4 +88,10 @@
       end
     end
   end
+
+  describe '#eager_load' do
+    it 'points to associations to be eager-loaded' do
+      expect(field.eager_load).to eq({active_storage_assets_attachments: :blob})
+    end
+  end
 end if defined?(ActiveStorage)