Demonstration of a race condition in MongoMapper without support for multiple atomic operators.

gistfile1.rb

class Company
  include MongoMapper::Document
  
  key :name, String
  
  key :bosses, Array
  key :employees, Array
  
  def promote!(employee)
    add_to_set(:bosses, employee)
    pull(:employee, employee)
  end
  
  def hassle_employee(employee)
    yeaaaah if employees.include?(employee)
  end
  
  def yeaaaah
    sleep 60 * 60 * 8
  end
end

Company.create(:name => "Initech", 
  :bosses => [:lumbergh, :dom], 
  :employees => [:peter, :michael, :samir, :milton])

# :the_bobs (not shown) want to promote :peter
# :lumbergh wants to hassle :peter
c1 = Company.find(:name => "Initech")
c1.promote!(:peter) #1
1 --> bosses.add_to_set(:bosses, :peter)
1 ----> db.collection("companies").update({:name => "Initech"}, 
  {"$addToSet" => {:bosses => "peter"}})
# :lumbergh (with a different Company object, since it could be in another request)
c2 = Company.find(:name => "Initech")
c2.hassle_employee(:peter) #2
2 --> employess.include?(:peter) => true
2 --> yeaaaah # Peter goes insane
# 2nd step of promote!(:peter)
1 --> employees.pull(:employees, :peter)
1 ----> db.collection("companies").update({:name => "Initech"}, 
  {"$pull" => {:employees => "peter"}})

class Company
  # Override old promote! with something that uses atomic updates
  def promote!(employee)
    modify do
      add_to_set(:bosses, employee)
      pull(:employees, employee)
    end
  end
end

# :lumbergh wants to hassle :peter
# :the_bobs (not shown) want to promote :peter
c1 = Company.find(:name => "Initech")
c1.promote!(:peter) #1
1 --> modify { add_to_set(:bosses, :peter); pull(:employees, :peter)}
1 ----> db.collection("companies").update({:name => "Initech"}, 
  {"$addToSet" => {:bosses => "peter"}, "$pull" => {:employees => "peter"})
# :lumbergh (with a different Company object, since it could be in another request)
c2 = Company.find(:name => "Initech")
c2.hassle_employee(:peter) #2
2 --> employees.include?(:peter) => false # Peter doesn't have to put up with Lumbergh anymore

See discussion: http://groups.google.com/group/mongomapper/browse_thread/thread/4b48720e5f5aec77