Use outer join for issues ordering by milestones due.

app/models/concerns/issuable.rb

@@ -35,13 +35,14 @@ module Issuable
     scope :only_opened, -> { with_state(:opened) }
     scope :only_reopened, -> { with_state(:reopened) }
     scope :closed, -> { with_state(:closed) }
-    scope :order_milestone_due_desc, -> { joins(:milestone).reorder('milestones.due_date DESC, milestones.id DESC') }
-    scope :order_milestone_due_asc, -> { joins(:milestone).reorder('milestones.due_date ASC, milestones.id ASC') }
+    scope :order_milestone_due_desc, -> { outer_join_milestone.reorder('milestones.due_date IS NULL ASC, milestones.due_date DESC, milestones.id DESC') }
+    scope :order_milestone_due_asc, -> { outer_join_milestone.reorder('milestones.due_date IS NULL ASC, milestones.due_date ASC, milestones.id ASC') }
     scope :without_label, -> { joins("LEFT OUTER JOIN label_links ON label_links.target_type = '#{name}' AND label_links.target_id = #{table_name}.id").where(label_links: { id: nil }) }
 
     scope :join_project, -> { joins(:project) }
     scope :references_project, -> { references(:project) }
     scope :non_archived, -> { join_project.where(projects: { archived: false }) }
+    scope :outer_join_milestone, -> { joins("LEFT OUTER JOIN milestones ON milestones.id = #{table_name}.milestone_id") }
 
     delegate :name,
              :email,

spec/features/issues_spec.rb

@@ -264,12 +264,14 @@ describe 'Issues', feature: true do
         visit namespace_project_issues_path(project.namespace, project, sort: sort_value_milestone_soon)
 
         expect(first_issue).to include('foo')
+        expect(last_issue).to include('baz')
       end
 
       it 'sorts by least recently due milestone' do
         visit namespace_project_issues_path(project.namespace, project, sort: sort_value_milestone_later)
 
         expect(first_issue).to include('bar')
+        expect(last_issue).to include('baz')
       end
     end