Use gitlab_subscription in group preloader

`plan_id` attribute on Group model is outdated and may
not be always set. Instead we should get plan id from
namespace's gitlab_subscription.

changelogs/unreleased/jprovazn-fix-group-preloader.yml

+---
+title: Fix loading of sub-epics caused by wrong subscription check.
+merge_request: 23184
+author:
+type: fixed

ee/app/models/ee/namespace.rb

@@ -35,6 +35,7 @@ module EE
       accepts_nested_attributes_for :gitlab_subscription
 
       scope :include_gitlab_subscription, -> { includes(:gitlab_subscription) }
+      scope :join_gitlab_subscription, -> { joins("LEFT OUTER JOIN gitlab_subscriptions ON gitlab_subscriptions.namespace_id=namespaces.id") }
       scope :with_plan, -> { where.not(plan_id: nil) }
       scope :with_shared_runners_minutes_limit, -> { where("namespaces.shared_runners_minutes_limit > 0") }
       scope :with_extra_shared_runners_minutes_limit, -> { where("namespaces.extra_shared_runners_minutes_limit > 0") }

ee/lib/gitlab/group_plans_preloader.rb

@@ -32,7 +32,7 @@ module Gitlab
 
           while current
 
-            if (plan_id = current.plan_id)
+            if (plan_id = current.hosted_plan_id)
               hash[group.id] << plan_id
               all_plan_ids << plan_id
             end
@@ -61,7 +61,8 @@ module Gitlab
       Gitlab::ObjectHierarchy
         .new(groups)
         .base_and_ancestors
-        .select(:id, :parent_id, :plan_id)
+        .join_gitlab_subscription
+        .select('namespaces.id', 'namespaces.parent_id', 'gitlab_subscriptions.hosted_plan_id')
     end
   end
 end

ee/spec/lib/gitlab/group_plans_preloader_spec.rb

@@ -14,9 +14,12 @@ describe Gitlab::GroupPlansPreloader do
     end
 
     before do
-      group1 = create(:group, name: 'group-1', plan_id: plan1.id)
+      group1 = create(:group, name: 'group-1')
+      create(:gitlab_subscription, namespace: group1, hosted_plan_id: plan1.id)
+
+      group2 = create(:group, name: 'group-2', plan_id: plan2.id)
+      create(:gitlab_subscription, namespace: group2, hosted_plan_id: plan2.id)
 
-      create(:group, name: 'group-2', plan_id: plan2.id)
       create(:group, name: 'group-3', parent: group1)
     end
 

ee/spec/models/namespace_spec.rb

@@ -77,6 +77,24 @@ describe Namespace do
         end
       end
     end
+
+    describe '.join_gitlab_subscription' do
+      subject { described_class.join_gitlab_subscription.select('gitlab_subscriptions.hosted_plan_id').first.hosted_plan_id }
+
+      context 'when there is no subscription' do
+        it 'returns namespace with nil subscription' do
+          is_expected.to be_nil
+        end
+      end
+
+      context 'when there is a subscription' do
+        let!(:subscription) { create(:gitlab_subscription, namespace: namespace, hosted_plan_id: gold_plan.id) }
+
+        it 'returns namespace with subscription set' do
+          is_expected.to eq(gold_plan.id)
+        end
+      end
+    end
   end
 
   context 'validation' do