Prevent more than one issue tracker to be active for the same project

app/controllers/projects/services_controller.rb

@@ -13,7 +13,8 @@ class Projects::ServicesController < Projects::ApplicationController
   end
 
   def update
-    if @service.update_attributes(service_params[:service])
+    @service.assign_attributes(service_params[:service])
+    if @service.save(context: :manual_change)
       redirect_to(
         edit_namespace_project_service_path(@project.namespace, @project, @service.to_param),
         notice: 'Successfully updated.'

app/models/project_services/issue_tracker_service.rb

 class IssueTrackerService < Service
+  validate :one_issue_tracker, if: :activated?, on: :manual_change
+
   default_value_for :category, 'issue_tracker'
 
   # Pattern used to extract links from comments
@@ -92,4 +94,13 @@ class IssueTrackerService < Service
   def issues_tracker
     Gitlab.config.issues_tracker[to_param]
   end
+
+  def one_issue_tracker
+    return if template?
+    return if project.blank?
+
+    if project.services.external_issue_trackers.where.not(id: id).any?
+      errors.add(:base, 'Another issue tracker is already in use. Only one issue tracker service can be active at a time')
+    end
+  end
 end

changelogs/unreleased/adam-prevent-two-issue-trackers.yml

+---
+title: Prevent more than one issue tracker to be active for the same project
+merge_request:
+author: luisdgs19

spec/models/project_services/issue_tracker_service_spec.rb

+require 'spec_helper'
+
+describe IssueTrackerService, models: true do
+  describe 'Validations' do
+    let(:project) { create :project }
+
+    describe 'only one issue tracker per project' do
+      let(:service) { RedmineService.new(project: project, active: true) }
+
+      before do
+        create(:service, project: project, active: true, category: 'issue_tracker')  
+      end
+
+      context 'when service is changed manually by user' do
+        it 'executes the validation' do
+          valid = service.valid?(:manual_change)
+
+          expect(valid).to be_falsey
+          expect(service.errors[:base]).to include(
+            'Another issue tracker is already in use. Only one issue tracker service can be active at a time'
+          )
+        end
+      end
+
+      context 'when service is changed internally' do
+        it 'does not execute the validation' do
+          expect(service.valid?).to be_truthy
+        end
+      end
+    end
+  end
+end