base_model.rb

module BaseModel
  def self.included(model)
    model.class_eval <<-RUBY, __FILE__, __LINE__ + 1
      include DataMapper::Resource

      property :id, Serial

      timestamps :at
    RUBY
  end
end

class Customer
  include BaseModel

  property :name, String

  has n, :orders
end

class Order
  include BaseModel

  property :reference_code, String

  belongs_to :customer
end