Include pagination when rendering expanded hierarchies

app/serializers/concerns/with_pagination.rb

 module WithPagination
+  attr_accessor :paginator
+
   def with_pagination(request, response)
-    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
+    tap { self.paginator = Gitlab::Serializer::Pagination.new(request, response) }
   end
 
   def paginated?
-    @paginator.present?
+    paginator.present?
   end
 
   # super is `BaseSerializer#represent` here.

app/serializers/group_child_serializer.rb

 class GroupChildSerializer < BaseSerializer
   include WithPagination
 
-  attr_reader :hierarchy_root
+  attr_reader :hierarchy_root, :should_expand_hierarchy
 
   entity GroupChildEntity
 
   def expand_hierarchy(hierarchy_root = nil)
+    tap do
       @hierarchy_root = hierarchy_root
-    @expand_hierarchy = true
-    self
+      @should_expand_hierarchy = true
+    end
   end
 
   def represent(resource, opts = {}, entity_class = nil)
-    if @expand_hierarchy
+    if should_expand_hierarchy
+      paginator.paginate(resource) if paginated?
       represent_hierarchies(resource, opts)
     else
       super(resource, opts)
@@ -33,15 +35,15 @@ class GroupChildSerializer < BaseSerializer
   def represent_hierarchy(hierarchy, opts)
     serializer = self.class.new(parameters)
 
-    result = if hierarchy.is_a?(Hash)
+    if hierarchy.is_a?(Hash)
       hierarchy.map do |parent, children|
         serializer.represent(parent, opts)
           .merge(children: Array.wrap(serializer.represent_hierarchy(children, opts)))
       end
+    elsif hierarchy.is_a?(Array)
+      hierarchy.map { |child| serializer.represent_hierarchy(child, opts) }
     else
       serializer.represent(hierarchy, opts)
     end
-
-    result
   end
 end

spec/controllers/groups_controller_spec.rb

@@ -152,6 +152,10 @@ describe GroupsController do
 
   describe 'GET #show' do
     context 'pagination' do
+      before do
+        allow(Kaminari.config).to receive(:default_per_page).and_return(2)
+      end
+
       context 'with only projects' do
         let!(:other_project) { create(:project, :public, namespace: group) }
         let!(:first_page_projects) { create_list(:project, Kaminari.config.default_per_page, :public, namespace: group ) }
@@ -288,6 +292,14 @@ describe GroupsController do
           expect(group_json['id']).to eq(public_subgroup.id)
           expect(matched_group_json['id']).to eq(matched_group.id)
         end
+
+        it 'includes pagination headers' do
+          2.times { |i| create(:group, :public, parent: public_subgroup, name: "filterme#{i}") }
+
+          get :children, id: group.to_param, filter: 'filter', per_page: 1, format: :json
+
+          expect(response).to include_pagination_headers
+        end
       end
 
       context 'queries per rendered element', :request_store do