signaling for notifiction api

This was intended to improve the performance of the most often used
request, the one to query for the existence of new notifications.
However, there is no performance gain to be measurable so that idea was
abandoned.

However, this at least fixes a bug the SQL based representers face when
no entries are returned by the selection of the resources listed in the
selection. Because it would result in an empty set, the expected json
would not be returned at all which then causes errors. The fix is
somewhat ugly but works.

lib/api/decorators/sql_collection_representer.rb

@@ -45,7 +45,10 @@ module API
         private
 
         def select_from(_walker_result)
-          "projection"
+          # The extra RIGHT JOIN is for the case where the results in `projection`
+          # are empty. If that is the case, the result set would be empty which would lead
+          # to nothing being returned, not even the collections metadata like count, total, ...
+          "projection RIGHT JOIN (VALUES (null)) AS t (placeholder) ON 1 = 1"
         end
 
         def full_self_path(walker_results, overrides = {})
@@ -91,7 +94,10 @@ module API
                representation: ->(*) { "'#{_type}'" }
 
       property :count,
-               representation: ->(*) { "COUNT(*)" }
+               # Because of the added placeholder RIGHT JOIN in the #select_from method,
+               # there will always be at last one column. So in case of not having any values,
+               # the calculation needs to be overridden.
+               representation: ->(*) { "CASE WHEN (SELECT * FROM total) > 0 THEN COUNT(*) ELSE 0 END" }
 
       property :total,
                representation: ->(*) { "(SELECT * from total)" }

lib/api/v3/notifications/notification_sql_collection_representer.rb

@@ -0,0 +1,35 @@
+# -- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) 2010-2023 the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+# ++
+
+module API
+  module V3
+    module Notifications
+      class NotificationSqlCollectionRepresenter < API::Decorators::SqlCollectionRepresenter; end
+    end
+  end
+end

lib/api/v3/notifications/notifications_api.rb

@@ -60,7 +60,7 @@ module API
             end
           end
 
-          get &::API::V3::Utilities::Endpoints::Index
+          get &::API::V3::Utilities::Endpoints::SqlFallbackedIndex
             .new(model: Notification, scope: -> { notification_scope })
             .mount
 

spec/lib/api/v3/projects/project_sql_collection_representer_rendering_spec.rb

@@ -142,4 +142,42 @@ describe API::V3::Projects::ProjectSqlCollectionRepresenter, 'rendering' do
         .to be_json_eql(expected)
     end
   end
+
+  context 'when not having a project to render' do
+    let(:scope) do
+      Project.none
+    end
+
+    let(:select) do
+      { '*' => {} }
+    end
+
+    let(:expected) do
+      {
+        _type: "Collection",
+        pageSize: 5,
+        total: 0,
+        count: 0,
+        offset: 1,
+        _links: {
+          self: {
+            href: "some_path?offset=1&pageSize=5&select=%2A"
+          },
+          changeSize: {
+            href: "some_path?offset=1&pageSize=%7Bsize%7D&select=%2A",
+            templated: true
+          },
+          jumpTo: {
+            href: "some_path?offset=%7Boffset%7D&pageSize=5&select=%2A",
+            templated: true
+          }
+        }
+      }.to_json
+    end
+
+    it 'renders as expected' do
+      expect(json)
+        .to be_json_eql(expected)
+    end
+  end
 end

spec/requests/api/v3/notifications/index_resource_spec.rb

@@ -341,6 +341,25 @@ describe API::V3::Notifications::NotificationsAPI,
 
       it_behaves_like 'API V3 collection response', 0, 0, 'Notification'
     end
+
+    context 'when signaling' do
+      let(:select) { 'total,count' }
+      let(:send_request) do
+        get api_v3_paths.path_for :notifications, select:
+      end
+
+      let(:expected) do
+        {
+          total: 2,
+          count: 2
+        }
+      end
+
+      it 'is the reduced set of properties of the embedded elements' do
+        expect(last_response.body)
+          .to be_json_eql(expected.to_json)
+      end
+    end
   end
 
   describe 'admin user' do