gistfile1.rb

%w[rubygems dm-core].each { |g| require g.to_s }

DataMapper.setup(:default, 'sqlite3::memory:')

class Something
  include DataMapper::Resource
  
  property :id, Serial
  property :p1, String
  property :p2, String
  
  attr_accessor :pv

end
DataMapper.auto_upgrade!

a = Something.new(:p1 => "property one", :p2 => "property 2", :pv => "property virtual")
a.save
# => true
b = Something.new(:p1 => "property one", :p2 => "property 2", :pv => "property virtual")
b.update
# => NoMethodError: undefined method  model for nil:NilClass