Don’t create new records on nested attributes

- When passing nested attributes for the gitlab_subscription record, we
shouldn’t create new records and instead update the existing (singleton)
record.

ee/app/models/ee/namespace.rb

@@ -31,7 +31,7 @@ module EE
       has_one :gitlab_subscription
       has_one :elasticsearch_indexed_namespace
 
-      accepts_nested_attributes_for :gitlab_subscription
+      accepts_nested_attributes_for :gitlab_subscription, update_only: true
       accepts_nested_attributes_for :namespace_limit
 
       scope :include_gitlab_subscription, -> { includes(:gitlab_subscription) }

ee/changelogs/unreleased/jj-adjust-subscription-association.yml

+---
+title: Don’t create new gitlab_subscription records on nested attributes to namespaces
+merge_request: 40537
+author:
+type: fixed

ee/spec/requests/api/namespaces_spec.rb

@@ -246,13 +246,14 @@ RSpec.describe API::Namespaces do
       end
 
       it "updates the gitlab_subscription record" do
-        group1.create_gitlab_subscription!
+        existing_subscription = group1.create_gitlab_subscription!
 
         put api("/namespaces/#{group1.id}", admin), params: {
           gitlab_subscription_attributes: gitlab_subscription
         }
 
         expect(group1.reload.gitlab_subscription.reload.seats).to eq 20
+        expect(group1.gitlab_subscription).to eq existing_subscription
       end
 
       context 'when params are invalid' do