class Company
  include DataMapper::Resource

  property :id,         Integer,  :serial   => true
  property :name,       String,   :nullable => false

  # on one side
  has n, :services, :child_key => [ :provider_id ]
end

class Service
  include DataMapper::Resource

  property :id,          Integer, :serial   => true
  property :name,        String,  :nullable => false, :index => true

  # and on the other, too
  belongs_to :provider, :class_name => "Company", :child_key => [ :provider_id ]
end