Add custom validator to runner model

app/controllers/projects/runners_controller.rb

@@ -20,7 +20,8 @@ def update
     if @runner.update_attributes(runner_params)
       redirect_to runner_path(@runner), notice: 'Runner was successfully updated.'
     else
-      redirect_to runner_path(@runner), alert: 'Runner was not updated.'
+      flash[:alert] = @runner.errors.full_messages.to_sentence
+      render 'edit'
     end
   end
 

app/models/ci/runner.rb

@@ -26,6 +26,13 @@ class Runner < ActiveRecord::Base
         .where("ci_runner_projects.gl_project_id = :project_id OR ci_runners.is_shared = true", project_id: project_id)
     end
 
+    validate do |runner|
+      if runner.tag_list.empty? && !runner.run_untagged?
+        errors.add(:tags_errors,
+          'Runner without tags must be able to pick untagged jobs!')
+      end
+    end
+
     acts_as_taggable
 
     # Searches for runners matching the given query.