def add_owners(hash, &factory)
      hash.keys.each { |klass| add_owners_for(klass, hash, &factory) }
    end
def add_owners(hash, &factory)
      hash.keys.each { |klass| add_owners_for(klass, hash, &factory) }
    end
Adds missing owner classes to the migration class path hash (with empty paths)
 for the classes in the given hash.

 @param [{Class => Object}] hash the class map
 @yield the map entry for a new owner