gistfile1.rb

# app/models/bar.rb
class Bar
  include DataMapper::Resource

  # property <name>, <type>
  property :id, Serial
  property :name, String
  # has many to many
  has n, :foos, :through => Resource
end

# app/models/foo.rb
class Foo
  include DataMapper::Resource

  # property <name>, <type>
  property :id, Serial
  property :name, String
  # has many to many
  has n, :bars, :through => Resource
end

# app/app.rb
get :index do
    # 1) create a first foo
    Foo.create(:name => "foo1")
    # create a three bar
    (1..3).each {|i| Bar.create(:name => "bar_#{i}") }

    # 2) get foo
    foo = Foo.first

    # 3) add bars to foo
    foo.bars = Bar.all(:id => [1,2])
    foo.save

    # 4) update foo
    foo.update(:name => "foo_update")

    # 5) update relationship  <<<<<  ERRROR WITH padrino "web"
    foo = Foo.first
    foo.bars = Bar.all(:id => 3)
    foo.save
    # when update foo.bars from web, dm not destroy the relationship with bar_id = 1,2
    # but if i use padrino console, dm work well
  end