Create notes for both issues when related

app/models/system_note_metadata.rb

@@ -2,8 +2,7 @@ class SystemNoteMetadata < ActiveRecord::Base
   ICON_TYPES = %w[
     commit description merge confidential visible label assignee cross_reference
     title time_tracking branch milestone discussion task moved opened closed merged
-    outdated
-    approved unapproved
+    outdated approved unapproved relate
   ].freeze
 
   validates :note, presence: true

app/services/related_issues/create_service.rb

@@ -23,11 +23,21 @@ module RelatedIssues
     def create_related_issues!
       RelatedIssue.transaction do
         referenced_issues.each do |referenced_issue|
-          RelatedIssue.create!(issue: @issue, related_issue: referenced_issue)
+          relate_issues!(referenced_issue)
+          create_notes!(referenced_issue)
         end
       end
     end
 
+    def relate_issues!(referenced_issue)
+      RelatedIssue.create!(issue: @issue, related_issue: referenced_issue)
+    end
+
+    def create_notes!(referenced_issue)
+      SystemNoteService.relate_issue(@issue, referenced_issue, current_user)
+      SystemNoteService.relate_issue(referenced_issue, @issue, current_user)
+    end
+
     def referenced_issues
       @referenced_issues ||= begin
         issue_references = params[:issue_references]

app/services/system_note_service.rb

@@ -552,6 +552,21 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, project, author, body, action: 'moved'))
   end
 
+  #
+  # noteable - Noteable object
+  # user     - User performing approve
+  #
+  # Example Note text:
+  #
+  #   "marked this issue as related to gitlab-ce#9001"
+  #
+  # Returns the created Note object
+  def relate_issue(noteable, noteable_ref, user)
+    body = "marked this issue as related to #{noteable_ref.to_reference(noteable.project)}"
+
+    create_note(NoteSummary.new(noteable, noteable.project, user, body, action: 'relate'))
+  end
+
   # Called when the merge request is approved by user
   #
   # noteable - Noteable object

spec/services/related_issues/create_service_spec.rb

@@ -72,6 +72,22 @@ describe RelatedIssues::CreateService, service: true do
       it 'returns success message with Issue references' do
         is_expected.to eq(message: "#{issue_a_ref} and #{another_project_issue_ref} were successfully related", status: :success)
       end
+
+      it 'creates notes' do
+        # First two-way relation notes
+        expect(SystemNoteService).to receive(:relate_issue)
+          .with(issue, issue_a, user)
+        expect(SystemNoteService).to receive(:relate_issue)
+          .with(issue_a, issue, user)
+
+        # Second two-way relation notes
+        expect(SystemNoteService).to receive(:relate_issue)
+          .with(issue, another_project_issue, user)
+        expect(SystemNoteService).to receive(:relate_issue)
+          .with(another_project_issue, issue, user)
+
+        subject
+      end
     end
 
     context 'when relation already exists' do

spec/services/system_note_service_spec.rb

@@ -899,6 +899,22 @@ describe SystemNoteService, services: true do
     end
   end
 
+  describe '.relate_issue' do
+    let(:noteable_ref) { create(:issue) }
+
+    subject { described_class.relate_issue(noteable, noteable_ref, author) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'relate' }
+    end
+
+    context 'when issue marks another as related' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "marked this issue as related to #{noteable_ref.to_reference(project)}"
+      end
+    end
+  end
+
   describe '.approve_mr' do
     let(:noteable)    { create(:merge_request, source_project: project) }
     subject { described_class.approve_mr(noteable, author) }