It is unable to migrate a database that was not yet seeded but containing some Puppetclasses (e.g. older version of Foreman).

Since the Rake task is simple, the solution is to copy the code into migration:

  desc "Correct hosts counts for all classes in case they are wrong" 
  task :fix_total_hosts => :environment do
    if Puppetclass.count > 0
      User.current = User.anonymous_admin
      Puppetclass.all.each(&:update_total_hosts)
    end
  end