MODEL:
class Gallery
  include DataMapper::Resource

  belongs_to :user
  has n, :photos

  def thumbnail
    "1"
  end
end

CONTROLLER:
def index
  #@galleries = User.first(:login => params[:login]).galleries # This doesn't work! DM bug?
  @galleries =  Gallery.all # This works!
  display @galleries
end

VIEW:
<%= partial 'galleries', :with => @galleries %>

PARTIAL:
<%= galleries.thumbnail %>