Fix missing and duplicates on project milestone listing page

app/models/concerns/issuable.rb

@@ -154,7 +154,7 @@ module Issuable
         end
 
       # Break ties with the ID column for pagination
-      sorted.order(id: :desc)
+      sorted.with_order_id_desc
     end
 
     def order_due_date_and_labels_priority(excluded_labels: [])

app/models/concerns/sortable.rb

@@ -6,6 +6,7 @@ module Sortable
   extend ActiveSupport::Concern
 
   included do
+    scope :with_order_id_desc, -> { order(id: :desc) }
     scope :order_id_desc, -> { reorder(id: :desc) }
     scope :order_id_asc, -> { reorder(id: :asc) }
     scope :order_created_desc, -> { reorder(created_at: :desc) }

app/models/milestone.rb

@@ -146,6 +146,7 @@ class Milestone < ActiveRecord::Base
   end
 
   def self.sort_by_attribute(method)
+    sorted =
       case method.to_s
       when 'due_date_asc'
         reorder(Gitlab::Database.nulls_last_order('due_date', 'ASC'))
@@ -162,6 +163,8 @@ class Milestone < ActiveRecord::Base
       else
         order_by(method)
       end
+
+    sorted.with_order_id_desc
   end
 
   ##

changelogs/unreleased/osw-fix-missing-and-duplicated-milestones-on-list.yml

+---
+title: Fix missing and duplicates on project milestone listing page
+merge_request: 21058
+author:
+type: fixed

spec/controllers/projects/milestones_controller_spec.rb

@@ -42,16 +42,45 @@ describe Projects::MilestonesController do
 
   describe "#index" do
     context "as html" do
-      before do
-        get :index, namespace_id: project.namespace.id, project_id: project.id
+      def render_index(project:, page:)
+        get :index, namespace_id: project.namespace.id,
+                    project_id: project.id,
+                    page: page
       end
 
       it "queries only projects milestones" do
+        render_index project: project, page: 1
+
         milestones = assigns(:milestones)
 
         expect(milestones.count).to eq(1)
         expect(milestones.where(project_id: nil)).to be_empty
       end
+
+      it 'renders paginated milestones without missing or duplicates' do
+        allow(Milestone).to receive(:default_per_page).and_return(2)
+        create_list(:milestone, 5, project: project)
+
+        render_index project: project, page: 1
+        page_1_milestones = assigns(:milestones)
+        expect(page_1_milestones.size).to eq(2)
+
+        render_index project: project, page: 2
+        page_2_milestones = assigns(:milestones)
+        expect(page_2_milestones.size).to eq(2)
+
+        render_index project: project, page: 3
+        page_3_milestones = assigns(:milestones)
+        expect(page_3_milestones.size).to eq(2)
+
+        rendered_milestone_ids =
+          page_1_milestones.pluck(:id) +
+          page_2_milestones.pluck(:id) +
+          page_3_milestones.pluck(:id)
+
+        expect(rendered_milestone_ids)
+          .to match_array(project.milestones.pluck(:id))
+      end
     end
 
     context "as json" do