Do not extend query for scope

app/models/queries/base_query.rb

@@ -38,8 +38,6 @@ class Queries::BaseQuery
   validate :filters_valid,
            :sortation_valid
 
-  attr_accessor :scope
-
   def initialize
     @scope = self.class.default_scope
     @filters = []
@@ -85,7 +83,9 @@ class Queries::BaseQuery
 
   protected
 
-  attr_accessor :filters, :orders
+  attr_accessor :scope,
+                :filters,
+                :orders
 
   def filters_valid
     filters.each do |filter|

app/services/api/v3/params_to_query_service.rb

@@ -29,20 +29,15 @@
 module API
   module V3
     class ParamsToQueryService
-      attr_accessor :model, :scope
+      attr_accessor :model
 
-      def initialize(model, scope: nil)
+      def initialize(model)
         self.model = model
-        self.scope = scope
       end
 
       def call(params)
         query = new_query
 
-        unless scope.nil?
-          query.scope = scope
-        end
-
         query = apply_filters(query, params)
         query = apply_order(query, params)
 

lib/api/v3/work_packages/watchers_api.rb

@@ -40,11 +40,10 @@ module API
           authorize(:add_work_package_watchers, context: @work_package.project)
 
           service = ::API::V3::ParamsToQueryService.new(User)
-          service.scope = @work_package.addable_watcher_users
           query = service.call(params)
 
           if query.valid?
-            users = query.results.includes(:preference)
+            users = query.results.merge(@work_package.addable_watcher_users).includes(:preference)
             ::API::V3::Users::PaginatedUserCollectionRepresenter.new(
               users,
               api_v3_paths.users,