main.rb

class Category
  include DataMapper::Resource

  property :id,         Serial
  property :name,       String

  has n, :notes
end

class Note
  include DataMapper::Resource

  property :id,         	Serial    # An auto-increment integer key
  property :category_id,	Integer   # An integer that acts as a foreign key
  property :title,      	String    # A varchar type string, for short strings
  property :body,       	Text      # A text block, for longer string data.
  property :created_at, 	DateTime  # A DateTime, for any date you might like.

  belongs_to :category
end

DataMapper.finalize
DataMapper.auto_upgrade!