Include identifier in Hierarchy eager loader's children SELECT

The ancestors/children representer change calls `child.display_id`,
which consults `identifier` in semantic mode. The Hierarchy eager
loader preloads children with a minimal `SELECT id, subject, project_id,
parent_id` for performance, so `identifier` was missing and
`ActiveModel::MissingAttributeError` fired the moment a query endpoint
rendered a work package with visible children in semantic mode.

Add `identifier` to the SELECT. It's one extra short text column per
child row.

lib/api/v3/work_packages/eager_loading/hierarchy.rb

@@ -41,7 +41,7 @@ module API
           def children(id)
             @children ||= WorkPackage
                           .where(parent_id: work_packages.map(&:id))
-                          .select(:id, :subject, :project_id, :parent_id)
+                          .select(:id, :identifier, :subject, :project_id, :parent_id)
                           .group_by(&:parent_id)
                           .to_h
 

spec/lib/api/v3/work_packages/eager_loading/hierarchy_integration_spec.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+require_relative "eager_loading_mock_wrapper"
+
+RSpec.describe API::V3::WorkPackages::EagerLoading::Hierarchy do
+  let!(:parent_work_package) { create(:work_package) }
+  let!(:child_work_package) { create(:work_package, parent: parent_work_package) }
+
+  describe ".apply" do
+    subject(:wrapped_children) do
+      wrapped = EagerLoadingMockWrapper.wrap(described_class, [parent_work_package])
+      wrapped.first.children
+    end
+
+    it "preloads the children association" do
+      wrapped = EagerLoadingMockWrapper.wrap(described_class, [parent_work_package])
+
+      expect(wrapped.first.association(:children)).to be_loaded
+      expect(wrapped_children.map(&:id)).to eq([child_work_package.id])
+    end
+
+    # Regression: children are loaded with a partial SELECT for performance,
+    # so the set of columns must cover everything the representer reads from
+    # each child — including `identifier`, which `display_id` consults to
+    # render the `_links.children[].displayId` payload in semantic mode.
+    it "includes identifier in the partial SELECT so display_id is available" do
+      child = wrapped_children.first
+
+      expect(child).to have_attribute(:identifier)
+      expect { child.display_id }.not_to raise_error
+    end
+  end
+end