what does an exclamation mark follow with variable mean in ruby on rails

Possible Duplicate:

Why are exclamation marks used in Ruby methods?

i am reading tutorials for Rails3 with MongoDB

http://www.mongodb.org/display/DOCS/MongoDB+Data+Modeling+and+Rails

and i see this
key :user_id, ObjectId
timestamps!
what does the exclamation mark mean??

Thanks.

 class Story
  include MongoMapper::Document

  key :title,     String
  key :url,       String
  key :slug,      String
  key :voters,    Array
  key :votes,     Integer, :default => 0
  key :relevance, Integer, :default => 0

  # Cached values.
  key :comment_count, Integer, :default => 0
  key :username,      String

  # Note this: ids are of class ObjectId.
  key :user_id,   ObjectId
  timestamps!

  # Relationships.
  belongs_to :user

  # Validations.
  validates_presence_of :title, :url, :user_id
end

in general, when a 'bang' follows a method in Ruby it will change the source.

for example check out the following output:

irb(main):007:0> x = 'string'
=> "string"
irb(main):008:0> x
=> "string"
irb(main):009:0> x.capitalize
=> "String"
irb(main):010:0> x
=> "string"
irb(main):011:0> x.capitalize!
=> "String"
irb(main):012:0> x
=> "String"

x.capitalize returns "String", but variable x remains lower case. When I add the ! ('bang') to the end var x is modified.

i am not overall familiar with mongodb, but this may shed some light onto the purpose of the bang in ruby.

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