# device.rb
class Device < Sequel::Model(:devices)
  validates_presence_of :device_type_id, :key => :id
  validates_presence_of :ipaddress
  validates_presence_of :name, :key => :id
end

# device_type.rb
require 'device'

class DeviceType < Sequel::Model(:device_types)
  validates do
    presence_of :vendor, :key => :id
    presence_of :model, :key => :id
    presence_of :is_su, :key => :id
  end

  def devices
    Device.where { :device_type_id == pk }
  end
end