Only preload ancestors for search results in the specified parent

When filtering we want all to preload all the ancestors upto the specified
parent group.

- root
  - subgroup
    - nested-group
      - project

So when searching 'project', on the 'subgroup' page we want to preload
'nested-group' but not 'subgroup' or 'root'

app/finders/group_descendants_finder.rb

@@ -96,9 +96,21 @@ class GroupDescendantsFinder
       .search(params[:filter])
   end
 
+  # When filtering we want all to preload all the ancestors upto the specified
+  # parent group.
+  #
+  # - root
+  #   - subgroup
+  #     - nested-group
+  #       - project
+  #
+  # So when searching 'project', on the 'subgroup' page we want to preload
+  # 'nested-group' but not 'subgroup' or 'root'
   def ancestors_for_groups(base_for_ancestors)
+    ancestors_for_parent = Gitlab::GroupHierarchy.new(Group.where(id: parent_group))
+                             .base_and_ancestors
     Gitlab::GroupHierarchy.new(base_for_ancestors)
-      .base_and_ancestors.where.not(id: parent_group)
+      .base_and_ancestors.where.not(id: ancestors_for_parent)
   end
 
   def subgroups

app/models/concerns/group_descendant.rb

@@ -14,7 +14,8 @@ module GroupDescendant
     if parent && parent != hierarchy_top
       expand_hierarchy_for_child(parent,
                                  { parent => hierarchy },
-                                 hierarchy_top)
+                                 hierarchy_top,
+                                 preloaded)
     else
       hierarchy
     end

spec/controllers/groups_controller_spec.rb

@@ -319,6 +319,16 @@ describe GroupsController do
           expect(matched_project_2_json['id']).to eq(matched_project_2.id)
         end
 
+        it 'expands the tree upto a specified parent' do
+          subgroup = create(:group, :public, parent: group)
+          l2_subgroup = create(:group, :public, parent: subgroup)
+          create(:project, :public, namespace: l2_subgroup, name: 'test')
+
+          get :children, id: subgroup.to_param, filter: 'test', format: :json
+
+          expect(response).to have_http_status(200)
+        end
+
         it 'includes pagination headers' do
           2.times { |i| create(:group, :public, parent: public_subgroup, name: "filterme#{i}") }