Fix filter on `dashboard/groups` & `explore/groups

When searching we would limit the scope of ancestors to load because
the filter would be applied to the ancestors. Instead, we need to load
_all_ ancestors for matching projects.

app/controllers/concerns/group_tree.rb

@@ -2,7 +2,11 @@ module GroupTree
   # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def render_group_tree(groups)
     @groups = if params[:filter].present?
-                Gitlab::GroupHierarchy.new(groups.search(params[:filter]))
+                # We find the ancestors by ID of the search results here.
+                # Otherwise the ancestors would also have filters applied,
+                # which would cause them not to be preloaded.
+                group_ids = groups.search(params[:filter]).select(:id)
+                Gitlab::GroupHierarchy.new(Group.where(id: group_ids))
                   .base_and_ancestors
               else
                 # Only show root groups if no parent-id is given

changelogs/unreleased/bvl-parent-preloading.yml

+---
+title: Fix issues when rendering groups and their children
+merge_request: 16584
+author:
+type: fixed

spec/controllers/dashboard/groups_controller_spec.rb

@@ -20,4 +20,24 @@ describe Dashboard::GroupsController do
     expect(assigns(:groups)).to contain_exactly(member_of_group)
   end
+
+  context 'when rendering an expanded hierarchy with public groups you are not a member of', :nested_groups do
+    let!(:top_level_result) { create(:group, name: 'chef-top') }
+    let!(:top_level_a) { create(:group, name: 'top-a') }
+    let!(:sub_level_result_a) { create(:group, name: 'chef-sub-a', parent: top_level_a) }
+    let!(:other_group) { create(:group, name: 'other') }
+
+    before do
+      top_level_result.add_master(user)
+      top_level_a.add_master(user)
+    end
+
+    it 'renders only groups the user is a member of when searching hierarchy correctly' do
+      get :index, filter: 'chef', format: :json
+
+      expect(response).to have_gitlab_http_status(200)
+      all_groups = [top_level_result, top_level_a, sub_level_result_a]
+      expect(assigns(:groups)).to contain_exactly(*all_groups)
+    end
+  end
 end