Add migration to fix not consistent data

And add specs to the migration

db/post_migrate/20201104124300_ensure_namespace_settings_creation.rb

+# frozen_string_literal: true
+
+class EnsureNamespaceSettingsCreation < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+  BATCH_SIZE = 10000
+
+  class Namespace < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'namespaces'
+  end
+
+  def up
+    ensure_data_migration
+  end
+
+  def down
+    # no-op
+  end
+
+  private
+
+  def ensure_data_migration
+    Namespace.each_batch(of: BATCH_SIZE) do |query|
+      missing_count = query.where("NOT EXISTS (SELECT 1 FROM namespace_settings WHERE namespace_settings.namespace_id=namespaces.id)").limit(1).size
+      if missing_count > 0
+        min, max = query.pluck("MIN(id), MAX(id)").flatten
+        # we expect low record count so inline execution is fine.
+        Gitlab::BackgroundMigration::BackfillNamespaceSettings.new.perform(min, max)
+      end
+    end
+  end
+end

db/schema_migrations/20201104124300

+e17da7eebb6d054a711368369d2b4fa684e96344f845bb7c6b3c89a9b4c4e067
\ No newline at end of file

db/structure.sql

@@ -22513,7 +22513,7 @@ ALTER INDEX product_analytics_events_experimental_pkey ATTACH PARTITION gitlab_p
 
 ALTER INDEX product_analytics_events_experimental_pkey ATTACH PARTITION gitlab_partitions_static.product_analytics_events_experimental_63_pkey;
 
-CREATE TRIGGER table_sync_trigger_ee39a25f9d AFTER INSERT OR DELETE OR UPDATE ON audit_events FOR EACH ROW EXECUTE PROCEDURE table_sync_function_2be879775d();
+CREATE TRIGGER table_sync_trigger_ee39a25f9d AFTER INSERT OR DELETE OR UPDATE ON audit_events FOR EACH ROW EXECUTE FUNCTION table_sync_function_2be879775d();
 
 ALTER TABLE ONLY chat_names
     ADD CONSTRAINT fk_00797a2bf9 FOREIGN KEY (service_id) REFERENCES services(id) ON DELETE CASCADE;

spec/migrations/ensure_namespace_settings_creation_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '/Users/gosia/gitlab-development-kit/gitlab/db/post_migrate/20201104124300_ensure_namespace_settings_creation.rb')
+
+RSpec.describe EnsureNamespaceSettingsCreation do
+  context 'when there are namespaces without namespace settings' do
+    let(:namespaces) { table(:namespaces) }
+    let(:namespace_settings) { table(:namespace_settings) }
+    let!(:namespace) { namespaces.create!(name: 'gitlab', path: 'gitlab-org') }
+
+    it 'migrates namespaces without namespace_settings' do
+      expect { migrate! }.to change { namespace_settings.count }.from(0).to(1)
+    end
+  end
+end