Fix weird Rails bug that leads to `runner_id=null` in SQL query

app/services/ci/register_job_service.rb

@@ -90,7 +90,7 @@ module Ci
 
     def builds_for_group_runner
       # Workaround for weird Rails bug, that makes `runner.groups.to_sql` to return `runner_id = NULL`
-      groups = Group.joins(:runner_namespaces).merge(runner.runner_namespaces)
+      groups = Group.joins(:runner_namespaces).where(runner_namespaces: { runner_id: runner })
       hierarchy_groups = Gitlab::GroupHierarchy.new(groups).base_and_descendants
       projects = Project.where(namespace_id: hierarchy_groups)
         .with_group_runners_enabled

spec/services/ci/register_job_service_spec.rb

@@ -5,15 +5,11 @@ module Ci
     set(:group) { create(:group) }
     set(:project) { create(:project, group: group, shared_runners_enabled: false, group_runners_enabled: false) }
     set(:pipeline) { create(:ci_pipeline, project: project) }
-    let!(:shared_runner) { create(:ci_runner, is_shared: true) }
-    let!(:specific_runner) { create(:ci_runner, is_shared: false) }
+    let!(:shared_runner) { create(:ci_runner, :instance) }
+    let!(:specific_runner) { create(:ci_runner, :project, projects: [project]) }
     let!(:group_runner) { create(:ci_runner, :group, groups: [group]) }
     let!(:pending_job) { create(:ci_build, pipeline: pipeline) }
 
-    before do
-      specific_runner.assign_to(project)
-    end
-
     describe '#execute' do
       context 'runner follow tag list' do
         it "picks build with the same tag" do