Demonstration of the bug at https://github.com/jnicklas/carrierwave/issues/issue/87

carrierwave_datamapper_bug.rb

require 'rubygems'
require 'sinatra'
require 'data_mapper'
require 'carrierwave'
require 'carrierwave/orm/datamapper'

ROOT = Dir.pwd

class ImageUploader < CarrierWave::Uploader::Base
  storage :file

  def store_dir
     nil  #store files at root level
  end

  def cache_dir
    "#{ROOT}/tmp/"
  end

end

class Person
  include DataMapper::Resource
  
  property :id, Serial
  property :name, String
  
  # http://github.com/jnicklas/carrierwave/issues#issue/87/comment/396035
  property :avatar, String, :length => 255, :auto_validation => false
  mount_uploader :avatar, ImageUploader

  # Override property setter to mark property as dirty
  #
  # Compare wprater's version (working):  
  # https://github.com/wprater/carrierwave/blob/datamapper_property_marked_dirty/lib/carrierwave/orm/datamapper.rb  
  #
  # with master version (not working): 
  # https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/orm/datamapper.rb
  #
  # Here's the relevant commit:
  # https://github.com/wprater/carrierwave/commit/f0245a66df097e78ab3bb77b4c6eb3018123109d

  #def avatar=(obj)
  #  self.attribute_set(:avatar, obj)
  #  _mounter(:avatar).cache(obj)
  #end

end

DataMapper::setup(:default, "sqlite3::memory:") 
DataMapper.auto_migrate! 
first_person = Person.new(:name => 'Alice')
if first_person.save
  puts "person saved successfully"
else
  first_person.errors.each do |e|
    puts e
  end
end

get '/' do
  erb :edit
end

post '/update' do
  person = Person.get(1)
  puts "POST /update"
  puts "params = #{params.inspect}"
  if person.update(params[:person])
    redirect '/'
  else
    person.errors.each do |e|
      puts e
    end   
  end
end

__END__

@@ edit
<h1>Carrierwave + Datamapper dirty bug</h1>
<p>Step 1: To demonstrate the bug, try uploading a new avatar image without modifying the person's name. The image will not be saved.</p>
<p>Step 2: Submit the form with a newly selected image and a modification to the person's name, and the image will be saved.</p>
<p>Step 3: To demonstrate the workaround, uncomment lines 43-46 and try Step 1 again. The image will be saved this time.</p>
<hr />
<form action="/update" method="POST" enctype="multipart/form-data">

  <p>Name:<br />
  <input type='text' name="person[name]" value="<%= Person.get(1).name %>" /></p>

  <p>Avatar:<br />
  <img src="<%= Person.get(1).avatar_url %>" /><br />
  <input type='file' name="person[avatar]" /></p>

  <p><input type='submit'/></p>

</form>