Improve `Ci::Runner#assign_to` to return a flag whether it succeeded or not

app/controllers/admin/runner_projects_controller.rb

@@ -4,9 +4,7 @@ class Admin::RunnerProjectsController < Admin::ApplicationController
   def create
     @runner = Ci::Runner.find(params[:runner_project][:runner_id])
 
-    runner_project = @runner.assign_to(@project, current_user)
-
-    if runner_project.persisted?
+    if @runner.assign_to(@project, current_user)
       redirect_to admin_runner_path(@runner)
     else
       redirect_to admin_runner_path(@runner), alert: 'Failed adding runner to project'

app/controllers/projects/runner_projects_controller.rb

@@ -9,9 +9,8 @@ class Projects::RunnerProjectsController < Projects::ApplicationController
     return head(403) unless can?(current_user, :assign_runner, @runner)
 
     path = project_runners_path(project)
-    runner_project = @runner.assign_to(project, current_user)
 
-    if runner_project.persisted?
+    if @runner.assign_to(project, current_user)
       redirect_to path
     else
       redirect_to path, alert: 'Failed adding runner to project'

app/models/ci/runner.rb

@@ -120,9 +120,8 @@ module Ci
         raise ArgumentError, 'Transitioning a group runner to a project runner is not supported'
       end
 
-      runner_project = project.runner_projects.create(runner_id: self.id)
-      self.save!
-      runner_project
+      self.projects << project
+      self.save
     end
 
     def display_name

lib/api/runners.rb

@@ -133,9 +133,7 @@ module API
         runner = get_runner(params[:runner_id])
         authenticate_enable_runner!(runner)
 
-        runner_project = runner.assign_to(user_project)
-
-        if runner_project.persisted?
+        if runner.assign_to(user_project)
           present runner, with: Entities::Runner
         else
           conflict!("Runner was already enabled for this project")

spec/models/ci/runner_spec.rb

@@ -200,7 +200,7 @@ describe Ci::Runner do
       let(:runner) { create(:ci_runner, :instance) }
 
       it 'transitions shared runner to project runner and assigns project' do
-        subject
+        expect(subject).to be_truthy
 
         expect(runner).to be_specific
         expect(runner).to be_project_type