Add activate_disabled_reason to models

We are passing a list of enabled trackers which can be used
by the frontend to inform the user of which ones are
already enabled (so they can disable them if needed).

app/models/integration.rb

@@ -346,6 +346,10 @@ class Integration < ApplicationRecord
     true
   end
 
+  def activate_disabled_reason
+    nil
+  end
+
   def category
     read_attribute(:category).to_sym
   end

app/models/integrations/base_issue_tracker.rb

@@ -132,8 +132,18 @@ module Integrations
       # implement inside child
     end
 
+    def activate_disabled_reason
+      { trackers: other_external_issue_trackers } if other_external_issue_trackers.any?
+    end
+
     private
 
+    def other_external_issue_trackers
+      return [] unless project_level?
+
+      project.integrations.external_issue_trackers.where.not(id: id)
+    end
+
     def enabled_in_gitlab_config
       Gitlab.config.issues_tracker &&
         Gitlab.config.issues_tracker.values.any? &&
@@ -148,7 +158,7 @@ module Integrations
       return if instance?
       return if project.blank?
 
-      if project.integrations.external_issue_trackers.where.not(id: id).any?
+      if other_external_issue_trackers.any?
         errors.add(:base, _('Another issue tracker is already in use. Only one issue tracker service can be active at a time'))
       end
     end

app/models/integrations/pivotaltracker.rb

@@ -71,8 +71,18 @@ module Integrations
       end
     end
 
+    def activate_disabled_reason
+      { trackers: external_issue_trackers } if external_issue_trackers.any?
+    end
+
     private
 
+    def external_issue_trackers
+      return false unless project_level?
+
+      project.integrations.external_issue_trackers
+    end
+
     def allowed_branch?(ref)
       return true unless ref.present? && restrict_to_branch.present?
 

spec/factories/integrations.rb

@@ -224,6 +224,12 @@ FactoryBot.define do
     recipients { 'test@example.com' }
   end
 
+  factory :pivotaltracker_integration, class: 'Integrations::Pivotaltracker' do
+    project
+    active { true }
+    token { 'test' }
+  end
+
   # this is for testing storing values inside properties, which is deprecated and will be removed in
   # https://gitlab.com/gitlab-org/gitlab/issues/29404
   trait :without_properties_callback do

spec/models/integrations/base_issue_tracker_spec.rb

@@ -3,12 +3,12 @@
 require 'spec_helper'
 
 RSpec.describe Integrations::BaseIssueTracker do
-  describe 'Validations' do
-    let(:project) { create :project }
-
-    describe 'only one issue tracker per project' do
   let(:integration) { Integrations::Redmine.new(project: project, active: true, issue_tracker_data: build(:issue_tracker_data)) }
 
+  let_it_be_with_refind(:project) { create :project }
+
+  describe 'Validations' do
+    describe 'only one issue tracker per project' do
       before do
         create(:custom_issue_tracker_integration, project: project)
       end
@@ -31,4 +31,18 @@ RSpec.describe Integrations::BaseIssueTracker do
       end
     end
   end
+
+  describe '#activate_disabled_reason' do
+    subject { integration.activate_disabled_reason }
+
+    context 'when there is an existing issue tracker integration' do
+      let_it_be(:custom_tracker) { create(:custom_issue_tracker_integration, project: project) }
+
+      it { is_expected.to eq(trackers: [custom_tracker]) }
+    end
+
+    context 'when there is no existing issue tracker integration' do
+      it { is_expected.to be(nil) }
+    end
+  end
 end

spec/models/integrations/pivotaltracker_spec.rb

@@ -93,4 +93,22 @@ RSpec.describe Integrations::Pivotaltracker do
       end
     end
   end
+
+  describe '#activate_disabled_reason' do
+    let(:integration) { build(:pivotaltracker_integration, project: project) }
+
+    let_it_be_with_refind(:project) { create :project }
+
+    subject { integration.activate_disabled_reason }
+
+    context 'when there is an external issue tracker integration' do
+      let_it_be(:custom_tracker) { create(:custom_issue_tracker_integration, project: project) }
+
+      it { is_expected.to eq(trackers: [custom_tracker]) }
+    end
+
+    context 'when there is no external issue tracker integration' do
+      it { is_expected.to be(nil) }
+    end
+  end
 end