gistfile1.rb

class Post
  include DataMapper::Resource

  property :id, Serial
  belongs_to :author, :default => lambda { |r, a| Author.first }
end


class Author
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  has n, :posts
end


bill = Author.create(:name => "Bill")
dave = Author.create(:name => "Dave")
post = Post.create(:author_id => dave.id)
puts post.author.name 

=> "Bill"
# default value of author (on post) is beating out the author_id passed in to create