gistfile1.rb

class Segment
  include DataMapper::Resource
  property :id,               Serial
  property :title,            String
  has n, :products, :through => Resource
end

class Product
  include DataMapper::Resource
  property :id,               Integer,  :serial => true
  property :title,            String
  property :permalink,        String
  has n, :segments, :through => Resource
end

Product form view:

<%= select :segments, :collection => @segments, :value_method => "id", :text_method => "title", :label => 'Segmento' %>
<%= text_field :title, :label => "Title" %>

Products controller

def create(product)
  @product = Product.new(product)
  @product.save
end

Ideally I'd like to have a select tag with segments[]

Error:
undefined method `collection=' for "1":String
gems/gems/dm-core-0.9.6/lib/dm-core/collection.rb in `relate_resource'   	 537