Add system note when due date is changed

app/helpers/system_note_helper.rb

@@ -21,7 +21,8 @@ module SystemNoteHelper
     'outdated' => 'pencil-square',
     'duplicate' => 'issue-duplicate',
     'locked' => 'lock',
-    'unlocked' => 'lock-open'
+    'unlocked' => 'lock-open',
+    'due_date' => 'calendar'
   }.freeze
 
   def system_note_icon_name(note)

app/models/system_note_metadata.rb

@@ -15,7 +15,7 @@ class SystemNoteMetadata < ActiveRecord::Base
     commit description merge confidential visible label assignee cross_reference
     title time_tracking branch milestone discussion task moved
     opened closed merged duplicate locked unlocked
-    outdated tag
+    outdated tag due_date
   ].freeze
 
   validates :note, presence: true

app/services/issuable/common_system_notes_service.rb

@@ -17,6 +17,7 @@ module Issuable
       create_labels_note(old_labels) if issuable.labels != old_labels
       create_discussion_lock_note if issuable.previous_changes.include?('discussion_locked')
       create_milestone_note if issuable.previous_changes.include?('milestone_id')
+      create_due_date_note if issuable.previous_changes.include?('due_date')
     end
 
     private
@@ -88,6 +89,10 @@ module Issuable
       SystemNoteService.change_milestone(issuable, issuable.project, current_user, issuable.milestone)
     end
 
+    def create_due_date_note
+      SystemNoteService.change_due_date(issuable, issuable.project, current_user, issuable.due_date)
+    end
+
     def create_discussion_lock_note
       SystemNoteService.discussion_lock(issuable, current_user)
     end

app/services/system_note_service.rb

@@ -160,6 +160,26 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, project, author, body, action: 'milestone'))
   end
 
+  # Called when the due_date of a Noteable is changed
+  #
+  # noteable  - Noteable object
+  # project   - Project owning noteable
+  # author    - User performing the change
+  # due_date  - Due date being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed due date"
+  #
+  #   "changed due date to September 20, 2018"
+  #
+  # Returns the created Note object
+  def change_due_date(noteable, project, author, due_date)
+    body = due_date ? "changed due date to #{due_date.to_s(:long)}" : 'removed due date'
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'due_date'))
+  end
+
   # Called when the estimated time of a Noteable is changed
   #
   # noteable      - Noteable object

changelogs/unreleased/38208-due-dates-system-notes.yml

+---
+title: Add system note when due date is changed
+merge_request:
+author: Eva Kadlecova
+type: added

spec/services/system_note_service_spec.rb

@@ -288,6 +288,30 @@ describe SystemNoteService do
     end
   end
 
+  describe '.change_due_date' do
+    subject { described_class.change_due_date(noteable, project, author, due_date) }
+
+    let(:due_date) { Date.today }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'due_date' }
+    end
+
+    context 'when due date added' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "changed due date to #{Date.today.to_s(:long)}"
+      end
+    end
+
+    context 'when due date removed' do
+      let(:due_date) { nil }
+
+      it 'sets the note text' do
+        expect(subject.note).to eq 'removed due date'
+      end
+    end
+  end
+
   describe '.change_status' do
     subject { described_class.change_status(noteable, project, author, status, source) }