class ActiveRecord::Associations::Builder::HasAndBelongsToMany

def belongs_to_options(options)
  rhs_options = {}
  if options.key? :class_name
    rhs_options[:foreign_key] = options[:class_name].to_s.foreign_key
    rhs_options[:class_name] = options[:class_name]
  end
  if options.key? :association_foreign_key
    rhs_options[:foreign_key] = options[:association_foreign_key]
  end
  rhs_options
end