Create RelatedIssues::DestroyService

app/services/related_issues/destroy_service.rb

+module RelatedIssues
+  class DestroyService < BaseService
+    def initialize(related_issue, user)
+      @related_issue = related_issue
+      @current_user = user
+      @issue = related_issue.issue
+      @referenced_issue = related_issue.related_issue
+    end
+
+    def execute
+      remove_relation!
+      create_notes!
+
+      success(message: 'Relation was removed')
+    end
+
+    private
+
+    def remove_relation!
+      @related_issue.destroy!
+    end
+
+    def create_notes!
+      SystemNoteService.unrelate_issue(@issue, @referenced_issue, current_user)
+      SystemNoteService.unrelate_issue(@referenced_issue, @issue, current_user)
+    end
+  end
+end

app/services/system_note_service.rb

@@ -554,7 +554,8 @@ module SystemNoteService
 
   #
   # noteable     - Noteable object
-  # user     - User performing approve
+  # noteable_ref - Referenced noteable object
+  # user         - User performing reference
   #
   # Example Note text:
   #
@@ -567,6 +568,22 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, noteable.project, user, body, action: 'relate'))
   end
 
+  #
+  # noteable     - Noteable object
+  # noteable_ref - Referenced noteable object
+  # user         - User performing reference
+  #
+  # Example Note text:
+  #
+  #   "removed the relation with gitlab-ce#9001"
+  #
+  # Returns the created Note object
+  def unrelate_issue(noteable, noteable_ref, user)
+    body = "removed the relation with #{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/destroy_service_spec.rb

+require 'spec_helper'
+
+describe RelatedIssues::DestroyService, service: true do
+  describe '#execute' do
+    let(:user) { create :user }
+    let!(:related_issue) { create :related_issue }
+
+    subject { described_class.new(related_issue, user).execute }
+
+    it 'remove related issue' do
+      expect { subject }.to change(RelatedIssue, :count).from(1).to(0)
+    end
+
+    it 'create notes' do
+      # Two-way notes creation
+      expect(SystemNoteService).to receive(:unrelate_issue)
+        .with(related_issue.issue, related_issue.related_issue, user)
+      expect(SystemNoteService).to receive(:unrelate_issue)
+        .with(related_issue.related_issue, related_issue.issue, user)
+
+      subject
+    end
+
+    it 'returns success message' do
+      is_expected.to eq(message: 'Relation was removed', status: :success)
+    end
+  end
+end

spec/services/system_note_service_spec.rb

@@ -915,6 +915,22 @@ describe SystemNoteService, services: true do
     end
   end
 
+  describe '.unrelate_issue' do
+    let(:noteable_ref) { create(:issue) }
+
+    subject { described_class.unrelate_issue(noteable, noteable_ref, author) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'unrelate' }
+    end
+
+    context 'when issue relation is removed' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "removed the relation with #{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) }