DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
On GitLab.com, there are over 2 million orphaned label links out of a total of 13 million. These orphaned label links can cause quiet failures, such as unexpected nil values in ExportCsvWorker. Closes gitlab-org/gitlab-ee#7482