dm_update_fk_bug.rb

#!/usr/bin/env ruby
#
# encoding: utf-8

require 'dm-sqlite-adapter'
require 'dm-migrations'
require 'dm-validations'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup :default, "sqlite::memory:"

class Ban
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  belongs_to :user # required by default
end

class User
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  has n, :bans
end

DataMapper.finalize
DataMapper.auto_migrate!

john = User.create(:name => "john")
jane = User.create(:name => "jane")

b = Ban.create(:name => "foo", :user => john)

b.user_id = jane.id
b.name    = "bar"

puts b.user_id.inspect # it is 2
puts b.save            # true
puts b.name            # *is* updated to 'bar'
puts b.user_id.inspect # but this is again 1!
puts b.user.inspect    # so this is still john!
puts b.errors.inspect  # no errors

patch.rb

module DataMapper
  module Associations
    module ManyToOne
      class Relationship < Associations::Relationship
        # @api semipublic
        def get!(resource)
          parent = resource.instance_variable_get(instance_variable_name)
          if parent_key.get!(parent) != source_key.get!(resource)
            parent = resource_for(resource)
          end
          parent
        end
      end
    end
  end
end