Fix relative siblings for epic tree nodes

Epic tree notes are polymorphic objects, and simply filtering on `id` is
not suitable for defining the set of siblings. This addresses this and
includes a reproducible test case.

ee/app/models/concerns/epic_tree_sorting.rb

@@ -19,13 +19,16 @@ module EpicTreeSorting
   end
 
   included do
+    extend ::Gitlab::Utils::Override
+
+    override :move_sequence
     def move_sequence(start_pos, end_pos, delta, include_self = false)
       items_to_update = scoped_items
         .select(:id, :object_type)
         .where('relative_position BETWEEN ? AND ?', start_pos, end_pos)
 
       unless include_self
-        items_to_update = items_to_update.where.not('object_type = ? AND id = ?', self.class.table_name.singularize, self.id)
+        items_to_update = relative_siblings(items_to_update)
       end
 
       items_to_update.group_by { |item| item.object_type }.each do |type, group_items|
@@ -34,5 +37,12 @@ module EpicTreeSorting
         items.update_all("relative_position = relative_position + #{delta}")
       end
     end
+
+    private
+
+    override :relative_siblings
+    def relative_siblings(relation = scoped_items)
+      relation.where.not('object_type = ? AND id = ?', self.class.table_name.singularize, self.id)
+    end
   end
 end

ee/changelogs/unreleased/ajk-relative-positioning-epic-sibs.yml

+---
+title: Correct sibling query for epic tree nodes
+merge_request: 41986
+author:
+type: fixed

ee/spec/models/concerns/epic_tree_sorting_spec.rb

@@ -12,6 +12,49 @@ RSpec.describe EpicTreeSorting do
   let!(:epic2) { create(:epic, parent: base_epic, group: group, relative_position: 1000) }
   let!(:epic3) { create(:epic, parent: base_epic, group: group, relative_position: 1001) }
 
+  describe '#relative_siblings' do
+    def siblings(obj)
+      obj.send(:relative_siblings).pluck(:id, :object_type)
+    end
+
+    def polymorphic_ident(obj)
+      case obj
+      when Epic
+        [obj.id, 'epic']
+      when EpicIssue
+        [obj.id, 'epic_issue']
+      end
+    end
+
+    it 'includes both epics and epic issues for an epic issue' do
+      idents = [epic_issue2, epic_issue3, epic1, epic2, epic3].map { |obj| polymorphic_ident(obj) }
+
+      expect(siblings(epic_issue1)).to match_array(idents)
+    end
+
+    it 'includes both epics and epic issues for an epic' do
+      idents = [epic_issue1, epic_issue2, epic_issue3, epic2, epic3].map { |obj| polymorphic_ident(obj) }
+
+      expect(siblings(epic1)).to match_array(idents)
+    end
+
+    context 'there is an ID collision' do
+      let(:max_epic_issue_id) { EpicIssue.maximum(:id) }
+      let(:max_epic_id) { Epic.maximum(:id) }
+
+      let(:collision_id) { [max_epic_id, max_epic_issue_id].max.succ }
+
+      it 'includes the collision from either collision member' do
+        colliding_epic = create(:epic, id: collision_id, parent: base_epic, group: group)
+        colliding_epic_issue = create(:epic_issue, id: collision_id, epic: base_epic)
+
+        expect(siblings(colliding_epic)).to include(polymorphic_ident(colliding_epic_issue))
+
+        expect(siblings(colliding_epic_issue)).to include(polymorphic_ident(colliding_epic))
+      end
+    end
+  end
+
   describe '#move_after' do
     it 'moves an epic' do
       epic1.move_after(epic_issue2)