ScratchPad.rb

include 'dm-is-list'

class Node
  include DataMapper::Resource

  property :id, Serial
  # other properties...

  has n, :parts
end

# Part is abstract, but should create a 'header' table
module Part
  def self.included(model)
    model.class_eval <<-RUBY, __FILE__, __LINE__ + 1
      include DataMapper::Resource

      property :id,       Serial
      property :subtitle, String, :nullable => false

      belongs_to :node

      is :list
    RUBY
  end
end

class PartText
  include Part

  property :data, Text, :nullable => false
end

class PartTable
  include Part

  property :caption, String
  property :cells,   Csv, :nullable => false

  # other properties
end

# Desired Usage
#
# table = PartTable.new
# table.caption = '...'
# table.cells = 'Some,csv'
# 
# text = PartText.new
# text.data = 'Really long passage of text'
#
# node = Node.new
# node.parts << table
# node.parts << text
# node.save