mimick id on non existent model

That way change detection can pick up setting an association with such a value

app/contracts/work_packages/base_contract.rb

@@ -320,7 +320,7 @@ module WorkPackages
     def validate_people_visible(attribute, id_attribute, list)
       id = model[id_attribute]
 
-      return if id.nil? || model.changed.exclude?(id_attribute)
+      return if id.nil? || id == 0 || model.changed.exclude?(id_attribute)
 
       unless principal_visible?(id, list)
         errors.add attribute,

app/models/concerns/inexistent_model.rb

@@ -0,0 +1,41 @@
+# --copyright
+# OpenProject is an open source project management software.
+# Copyright (C) 2010-2022 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 InexistentModel
+  extend ActiveSupport::Concern
+
+  included do
+    # Set an id so that e.g. change detection (dirty) on associations
+    # is picked up whenever such a model is assigned.
+    after_initialize do
+      self.id = 0
+    end
+
+    _validators.clear
+  end
+end

app/models/priority/inexistent_priority.rb

@@ -26,4 +26,6 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-class Priority::InexistentPriority < IssuePriority; end
+class Priority::InexistentPriority < IssuePriority
+  include InexistentModel
+end

app/models/status/inexistent_status.rb

@@ -26,4 +26,6 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-class Status::InexistentStatus < Status; end
+class Status::InexistentStatus < Status
+  include InexistentModel
+end

app/models/type/inexistent_type.rb

@@ -27,4 +27,5 @@
 #++
 
 class Type::InexistentType < Type
+  include InexistentModel
 end

app/models/users/inexistent_user.rb

@@ -27,6 +27,8 @@
 #++
 
 class Users::InexistentUser < User
+  include InexistentModel
+
   def self.sti_name
     nil
   end

app/models/work_package/inexistent_work_package.rb

@@ -27,7 +27,7 @@
 #++
 
 class WorkPackage::InexistentWorkPackage < WorkPackage
-  _validators.clear
+  include InexistentModel
 
   def does_not_exist
     errors.add :base, :does_not_exist

modules/storages/app/models/storages/storage/inexistent_storage.rb

@@ -27,5 +27,5 @@
 #++
 
 class Storages::Storage::InexistentStorage < Storages::Storage
-  _validators.clear
+  include InexistentModel
 end