Fix crashing in-product-marketing email worker

This is a temporary fix for the in-product-marketing-email worker.
It is a bug that consists of multiple layers. We expect to only have
root namespaces in `onboarding_progresses`. However, groups can be moved
to sub-groups. Threfore we also have sub-groups that would receieve the
emails.

This alone would not be an issue. But we need to call `.root_ancestor`
for these sub-groups and it doesn't play well together with
`each_batches` when there is an other JOIN condition on the namespace.
It currently generates a non-working query that fails.

To prevent this for now we do not send emails to sub-groups as a
temporary solution. In the long run, we need to remove all sub-groups
fron `onboarding_progresses` and fix the issue of calling
`.root_ancestor` in an `each_batches` call.

app/services/namespaces/in_product_marketing_emails_service.rb

@@ -63,7 +63,10 @@ module Namespaces
         .completed_actions_with_latest_in_range(completed_actions, range)
         .incomplete_actions(incomplete_action)
 
-      Group.joins(:onboarding_progress).merge(onboarding_progress_scope)
+      # Filtering out sub-groups is a temporary fix to prevent calling
+      # `.root_ancestor` on groups that are not root groups.
+      # See https://gitlab.com/groups/gitlab-org/-/epics/5594 for more information.
+      Group.where(parent_id: nil).joins(:onboarding_progress).merge(onboarding_progress_scope)
     end
 
     def users_for_group(group)

spec/services/namespaces/in_product_marketing_emails_service_spec.rb

@@ -159,4 +159,20 @@ RSpec.describe Namespaces::InProductMarketingEmailsService, '#execute' do
 
     it { expect { subject }.to raise_error(NotImplementedError, 'No ability defined for track foo') }
   end
+
+  context 'when group is a sub-group' do
+    let(:root_group) { create(:group) }
+    let(:group) { create(:group) }
+
+    before do
+      group.parent = root_group
+      group.save!
+
+      allow(Ability).to receive(:allowed?).and_call_original
+    end
+
+    it 'does not raise an exception' do
+      expect { execute_service }.not_to raise_error
+    end
+  end
 end