Always preload all elements in a hierarchy to avoid extra queries.

app/models/concerns/group_descendant.rb

 module GroupDescendant
-  def hierarchy(hierarchy_top = nil, preloaded = [])
+  def hierarchy(hierarchy_top = nil, preloaded = nil)
+    preloaded ||= ancestors_upto(hierarchy_top)
     expand_hierarchy_for_child(self, self, hierarchy_top, preloaded)
   end
 
@@ -24,12 +25,20 @@ module GroupDescendant
 
   private
 
-  def expand_hierarchy_for_child(child, hierarchy, hierarchy_top, preloaded = [])
+  def ancestors_upto(hierarchy_top = nil)
+    if hierarchy_top
+      Gitlab::GroupHierarchy.new(Group.where(id: hierarchy_top)).base_and_descendants
+    else
+      Gitlab::GroupHierarchy.new(Group.where(id: self)).all_groups
+    end
+  end
+
+  def expand_hierarchy_for_child(child, hierarchy, hierarchy_top, preloaded)
     parent = preloaded.detect { |possible_parent| possible_parent.is_a?(Group) && possible_parent.id == child.parent_id }
     parent ||= child.parent
 
     if parent.nil? && hierarchy_top.present?
-      raise ArgumentError.new('specified base is not part of the tree')
+      raise ArgumentError.new('specified top is not part of the tree')
     end
 
     if parent && parent != hierarchy_top

spec/models/concerns/group_descendant_spec.rb

@@ -7,6 +7,23 @@ describe GroupDescendant, :nested_groups do
 
   context 'for a group' do
     describe '#hierarchy' do
+      it 'only queries once for the ancestors' do
+        # make sure the subsub_group does not have anything cached
+        test_group = create(:group, parent: subsub_group).reload
+
+        query_count = ActiveRecord::QueryRecorder.new { test_group.hierarchy }.count
+
+        expect(query_count).to eq(1)
+      end
+
+      it 'only queries once for the ancestors when a top is given' do
+        test_group = create(:group, parent: subsub_group).reload
+
+        query_count = ActiveRecord::QueryRecorder.new { test_group.hierarchy(subgroup) }.count
+
+        expect(query_count).to eq(1)
+      end
+
       it 'builds a hierarchy for a group' do
         expected_hierarchy = { parent => { subgroup => subsub_group } }
 
@@ -20,7 +37,7 @@ describe GroupDescendant, :nested_groups do
       end
 
       it 'raises an error if specifying a base that is not part of the tree' do
-        expect { subsub_group.hierarchy(double) }.to raise_error('specified base is not part of the tree')
+        expect { subsub_group.hierarchy(build_stubbed(:group)) }.to raise_error('specified top is not part of the tree')
       end
     end
 
@@ -77,7 +94,7 @@ describe GroupDescendant, :nested_groups do
       end
 
       it 'raises an error if specifying a base that is not part of the tree' do
-        expect { project.hierarchy(double) }.to raise_error('specified base is not part of the tree')
+        expect { project.hierarchy(build_stubbed(:group)) }.to raise_error('specified top is not part of the tree')
       end
     end