PaperTrail

PaperTrail lets you track changes to your models' data. It's good for auditing or versioning. You can see how a model looked at any stage in its lifecycle, revert it to any version, and even undelete it after it's been destroyed.

There's an excellent Railscast on implementing Undo with Paper Trail.

Features

• Stores every create, update and destroy.
• Does not store updates which don't change anything.
• Allows you to specify attributes (by inclusion or exclusion) which must change for a Version to be stored.
• Allows you to get at every version, including the original, even once destroyed.
• Allows you to get at every version even if the schema has since changed.
• Allows you to get at the version as of a particular time.
• Option to automatically restore has_one associations as they were at the time.
• Automatically records who was responsible via your controller. PaperTrail calls current_user by default, if it exists, but you can have it call any method you like.
• Allows you to set who is responsible at model-level (useful for migrations).
• Allows you to store arbitrary model-level metadata with each version (useful for filtering versions).
• Allows you to store arbitrary controller-level information with each version, e.g. remote IP.
• Can be turned off/on per class (useful for migrations).
• Can be turned off/on per request (useful for testing with an external service).
• Can be turned off/on globally (useful for testing).
• No configuration necessary.
• Stores everything in a single database table by default (generates migration for you), or can use separate tables for separate models.
• Supports custom version classes so different models' versions can have different behaviour.
• Thoroughly tested.
• Threadsafe.

Rails Version

Works on Rails 3 and Rails 2.3. The Rails 3 code is on the master branch and tagged v2.x. The Rails 2.3 code is on the rails2 branch and tagged v1.x. Please note I'm not adding new features to the Rails 2.3 codebase.

API Summary

When you declare has_paper_trail in your model, you get these methods:

class Widget < ActiveRecord::Base
  has_paper_trail   # you can pass various options here
end

# Returns this widget's versions.
widget.versions

# Return the version this widget was reified from, or nil if it is live.
widget.version

# Returns true if this widget is the current, live one; or false if it is from a previous version.
widget.live?

# Returns who put the widget into its current state.
widget.originator

# Returns the widget (not a version) as it looked at the given timestamp.
widget.version_at(timestamp)

# Returns the widget (not a version) as it was most recently.
widget.previous_version

# Returns the widget (not a version) as it became next.
widget.next_version

# Turn PaperTrail off for all widgets.
Widget.paper_trail_off

# Turn PaperTrail on for all widgets.
Widget.paper_trail_on

And a Version instance has these methods:

# Returns the item restored from this version.
version.reify(options = {})

# Returns who put the item into the state stored in this version.
version.originator

# Returns who changed the item from the state it had in this version.
version.terminator
version.whodunnit

# Returns the next version.
version.next

# Returns the previous version.
version.previous

# Returns the index of this version in all the versions.
version.index

# Returns the event that caused this version (create|update|destroy).
version.event

In your controllers you can override these methods:

# Returns the user who is responsible for any changes that occur.
# Defaults to current_user.
user_for_paper_trail

# Returns any information about the controller or request that you want
# PaperTrail to store alongside any changes that occur.
info_for_paper_trail

Basic Usage

PaperTrail is simple to use. Just add 15 characters to a model to get a paper trail of every create, update, and destroy.

class Widget < ActiveRecord::Base
  has_paper_trail
end

This gives you a versions method which returns the paper trail of changes to your model.

>> widget = Widget.find 42
>> widget.versions             # [<Version>, <Version>, ...]

Once you have a version, you can find out what happened:

>> v = widget.versions.last
>> v.event                     # 'update' (or 'create' or 'destroy')
>> v.whodunnit                 # '153'  (if the update was via a controller and
                               #         the controller has a current_user method,
                               #         here returning the id of the current user)
>> v.created_at                # when the update occurred
>> widget = v.reify            # the widget as it was before the update;
                               # would be nil for a create event

PaperTrail stores the pre-change version of the model, unlike some other auditing/versioning plugins, so you can retrieve the original version. This is useful when you start keeping a paper trail for models that already have records in the database.

>> widget = Widget.find 153
>> widget.name                                 # 'Doobly'

# Add has_paper_trail to Widget model.

>> widget.versions                             # []
>> widget.update_attributes :name => 'Wotsit'
>> widget.versions.first.reify.name            # 'Doobly'
>> widget.versions.first.event                 # 'update'

This also means that PaperTrail does not waste space storing a version of the object as it currently stands. The versions method gives you previous versions; to get the current one just call a finder on your Widget model as usual.

Here's a helpful table showing what PaperTrail stores:

Event	Model Before	Model After
create	nil	widget
update	widget	widget'
destroy	widget	nil

PaperTrail stores the values in the Model Before column. Most other auditing/versioning plugins store the After column.

Choosing Attributes To Monitor

You can ignore changes to certain attributes like this:

class Article < ActiveRecord::Base
  has_paper_trail :ignore => [:title, :rating]
end

This means that changes to just the title or rating will not store another version of the article. It does not mean that the title and rating attributes will be ignored if some other change causes a new Version to be crated. For example:

>> a = Article.create
>> a.versions.length                         # 1
>> a.update_attributes :title => 'My Title', :rating => 3
>> a.versions.length                         # 1
>> a.update_attributes :content => 'Hello'
>> a.versions.length                         # 2
>> a.versions.last.reify.title               # 'My Title'

Or, you can specify a list of all attributes you care about:

class Article < ActiveRecord::Base
  has_paper_trail :only => [:title]
end

This means that only changes to the title will save a version of the article:

>> a = Article.create
>> a.versions.length                         # 1
>> a.update_attributes :title => 'My Title'
>> a.versions.length                         # 2
>> a.update_attributes :content => 'Hello'
>> a.versions.length                         # 2

Passing both :ignore and :only options will result in the article being saved if a changed attribute is included in :only but not in :ignore.

Reverting And Undeleting A Model

PaperTrail makes reverting to a previous version easy:

>> widget = Widget.find 42
>> widget.update_attributes :name => 'Blah blah'
# Time passes....
>> widget = widget.versions.last.reify  # the widget as it was before the update
>> widget.save                          # reverted

Alternatively you can find the version at a given time:

>> widget = widget.version_at(1.day.ago)  # the widget as it was one day ago
>> widget.save                            # reverted

Note version_at gives you the object, not a version, so you don't need to call reify.

Undeleting is just as simple:

>> widget = Widget.find 42
>> widget.destroy
# Time passes....
>> widget = Version.find(153).reify    # the widget as it was before it was destroyed
>> widget.save                         # the widget lives!

In fact you could use PaperTrail to implement an undo system, though I haven't had the opportunity yet to do it myself. However Ryan Bates has!

Navigating Versions

You can call previous_version and next_version on an item to get it as it was/became. Note that these methods reify the item for you.

>> widget = Widget.find 42
>> widget.versions.length              # 4 for example
>> widget = widget.previous_version    # => widget == widget.versions.last.reify
>> widget = widget.previous_version    # => widget == widget.versions[-2].reify
>> widget.next_version                 # => widget == widget.versions.last.reify
>> widget.next_version                 # nil

As an aside, I'm undecided about whether widget.versions.last.next_version should return nil or self (i.e. widget). Let me know if you have a view.

If instead you have a particular version of an item you can navigate to the previous and next versions.

>> widget = Widget.find 42
>> version = widget.versions[-2]    # assuming widget has several versions
>> previous = version.previous
>> next = version.next

You can find out which of an item's versions yours is:

>> current_version_number = version.index    # 0-based

Finally, if you got an item by reifying one of its versions, you can navigate back to the version it came from:

>> latest_version = Widget.find(42).versions.last
>> widget = latest_version.reify
>> widget.version == latest_version    # true

You can find out whether a model instance is the current, live one -- or whether it came instead from a previous version -- with live?:

>> widget = Widget.find 42
>> widget.live?                        # true
>> widget = widget.versions.last.reify
>> widget.live?                        # false

Finding Out Who Was Responsible For A Change

If your ApplicationController has a current_user method, PaperTrail will store the value it returns in the version's whodunnit column. Note that this column is a string so you will have to convert it to an integer if it's an id and you want to look up the user later on:

>> last_change = Widget.versions.last
>> user_who_made_the_change = User.find last_change.whodunnit.to_i

You may want PaperTrail to call a different method to find out who is responsible. To do so, override the user_for_paper_trail method in your controller like this:

class ApplicationController
  def user_for_paper_trail
    logged_in? ? current_member : 'Public user'  # or whatever
  end
end

In a migration or in script/console you can set who is responsible like this:

>> PaperTrail.whodunnit = 'Andy Stewart'
>> widget.update_attributes :name => 'Wibble'
>> widget.versions.last.whodunnit              # Andy Stewart

N.B. A version's whodunnit records who changed the object causing the version to be stored. Because a version stores the object as it looked before the change (see the table above), whodunnit returns who stopped the object looking like this -- not who made it look like this. Hence whodunnit is aliased as terminator.

To find out who made a version's object look that way, use version.originator. And to find out who made a "live" object look like it does, use originator on the object.

>> widget = Widget.find 153                    # assume widget has 0 versions
>> PaperTrail.whodunnit = 'Alice'
>> widget.update_attributes :name => 'Yankee'
>> widget.originator                           # 'Alice'
>> PaperTrail.whodunnit = 'Bob'
>> widget.update_attributes :name => 'Zulu'
>> widget.originator                           # 'Bob'
>> first_version, last_version = widget.versions.first, widget.versions.last
>> first_version.whodunnit                     # 'Alice'
>> first_version.originator                    # nil
>> first_version.terminator                    # 'Alice'
>> last_version.whodunnit                      # 'Bob'
>> last_version.originator                     # 'Alice'
>> last_version.terminator                     # 'Bob'

Custom Version Classes

You can specify custom version subclasses with the :class_name option:

class Post < ActiveRecord::Base
  has_paper_trail :class_name => 'PostVersion'
end

class PostVersion < Version
  # custom behaviour, e.g:
  set_table_name :post_versions
end

This allows you to store each model's versions in a separate table, which is useful if you have a lot of versions being created.

Alternatively you could store certain metadata for one type of version, and other metadata for other versions.

Associations

I haven't yet found a good way to get PaperTrail to automatically restore associations when you reify a model. See here for a little more info.

If you can think of a good way to achieve this, please let me know.

Has-One Associations

PaperTrail can restore :has_one associations as they were at (actually, 3 seconds before) the time.

class Treasure < ActiveRecord::Base
  has_one :location
end

>> treasure.amount                  # 100
>> treasure.location.latitude       # 12.345

>> treasure.update_attributes :amount => 153
>> treasure.location.update_attributes :latitude => 54.321

>> t = treasure.versions.last.reify(:has_one => true)
>> t.amount                         # 100
>> t.location.latitude              # 12.345

The implementation is complicated by the edge case where the parent and child are updated in one go, e.g. in one web request or database transaction. PaperTrail doesn't know about different models being updated "together", so you can't ask it definitively to get the child as it was before the joint parent-and-child update.

The correct solution is to make PaperTrail aware of requests or transactions (c.f. Efficiency's transaction ID middleware). In the meantime we work around the problem by finding the child as it was a few seconds before the parent was updated. By default we go 3 seconds before but you can change this by passing the desired number of seconds to the :has_one option:

>> t = treasure.versions.last.reify(:has_one => 1)       # look back 1 second instead of 3

If you are shuddering, take solace from knowing PaperTrail opts out of these shenanigans by default. This means your :has_one associated objects will be the live ones, not the ones the user saw at the time. Since PaperTrail doesn't auto-restore :has_many associations (I can't get it to work) or :belongs_to (I ran out of time looking at :has_many), this at least makes your associations wrong consistently ;)

Has-Many-Through Associations

PaperTrail can track most changes to the join table. Specifically it can track all additions but it can only track removals which fire the after_destroy callback on the join table. Here are some examples:

Given these models:

class Book < ActiveRecord::Base
  has_many :authorships, :dependent => :destroy
  has_many :authors, :through => :authorships, :source => :person
  has_paper_trail
end

class Authorship < ActiveRecord::Base
  belongs_to :book
  belongs_to :person
  has_paper_trail      # NOTE
end

class Person < ActiveRecord::Base
  has_many :authorships, :dependent => :destroy
  has_many :books, :through => :authorships
  has_paper_trail
end

Then each of the following will store authorship versions:

>> @book.authors << @dostoyevsky
>> @book.authors.create :name => 'Tolstoy'
>> @book.authorships.last.destroy
>> @book.authorships.clear

But none of these will:

>> @book.authors.delete @tolstoy
>> @book.author_ids = [@solzhenistyn.id, @dostoyevsky.id]
>> @book.authors = []

Having said that, you can apparently get all these working (I haven't tested it myself) with this monkey patch:

# In config/initializers/core_extensions.rb or lib/core_extensions.rb
ActiveRecord::Associations::HasManyThroughAssociation.class_eval do 
  def delete_records(records)
    klass = @reflection.through_reflection.klass
    records.each do |associate|
      klass.destroy_all(construct_join_attributes(associate))
    end
  end
end

The difference is the call to destroy_all instead of delete_all in the original.

There may be a way to store authorship versions, probably using association callbacks, no matter how the collection is manipulated but I haven't found it yet. Let me know if you do.

Storing metadata

You can store arbitrary model-level metadata alongside each version like this:

class Article < ActiveRecord::Base
  belongs_to :author
  has_paper_trail :meta => { :author_id  => Proc.new { |article| article.author_id },
                             :word_count => :count_words,
                             :answer     => 42 }
  def count_words
    153
  end
end

PaperTrail will call your proc with the current article and store the result in the author_id column of the versions table. (Remember to add your metadata columns to the table.)

Why would you do this? In this example, author_id is an attribute of Article and PaperTrail will store it anyway in serialized (YAML) form in the object column of the version record. But let's say you wanted to pull out all versions for a particular author; without the metadata you would have to deserialize (reify) each version object to see if belonged to the author in question. Clearly this is inefficient. Using the metadata you can find just those versions you want:

Version.all(:conditions => ['author_id = ?', author_id])

Note you can pass a symbol as a value in the meta hash to signal a method to call.

You can also store any information you like from your controller. Just override the info_for_paper_trail method in your controller to return a hash whose keys correspond to columns in your versions table. E.g.:

class ApplicationController
  def info_for_paper_trail
    { :ip => request.remote_ip, :user_agent => request.user_agent }
  end
end

Remember to add those extra columns to your versions table ;)

Diffing Versions

When you're storing every version of an object, as PaperTrail lets you do, you're almost certainly going to want to diff those versions against each other. However I haven't built a diff method into PaperTrail because I think diffing is best left to dedicated libraries, and also it's hard to come up with a diff method to suit all the use cases.

You might be surprised that PaperTrail doesn't use diffs internally anyway. When I designed PaperTrail I wanted simplicity and robustness so I decided to make each version of an object self-contained. A version stores all of its object's data, not a diff from the previous version.

So instead here are some specialised diffing libraries which you can use on top of PaperTrail.

For diffing two strings:

• htmldiff: expects but doesn't require HTML input and produces HTML output. Works very well but slows down significantly on large (e.g. 5,000 word) inputs.
• differ: expects plain text input and produces plain text/coloured/HTML/any output. Can do character-wise, word-wise, line-wise, or arbitrary-boundary-string-wise diffs. Works very well on non-HTML input.
• diff-lcs: old-school, line-wise diffs.

For diffing two ActiveRecord objects:

• Jeremy Weiskotten's PaperTrail fork: uses ActiveSupport's diff to return an array of hashes of the changes.
• activerecord-diff: rather like ActiveRecord::Dirty but also allows you to specify which columns to compare.

Turning PaperTrail Off/On

Sometimes you don't want to store changes. Perhaps you are only interested in changes made by your users and don't need to store changes you make yourself in, say, a migration -- or when testing your application.

You can turn PaperTrail on or off in three ways: globally, per request, or per class.

Globally

On a global level you can turn PaperTrail off like this:

>> PaperTrail.enabled = false

For example, you might want to disable PaperTrail in your Rails application's test environment to speed up your tests. This will do it:

# in config/environments/test.rb
config.after_initialize do
  PaperTrail.enabled = false
end

If you disable PaperTrail in your test environment but want to enable it for specific tests, you can add a helper like this to your test helper:

# in test/test_helper.rb
def with_versioning
  was_enabled = PaperTrail.enabled?
  PaperTrail.enabled = true
  begin
    yield
  ensure
    PaperTrail.enabled = was_enabled
  end
end

And then use it in your tests like this:

test "something that needs versioning" do
  with_versioning do
    # your test
  end
end

Per request

You can turn PaperTrail on or off per request by adding a paper_trail_enabled_for_controller method to your controller which returns true or false:

class ApplicationController < ActionController::Base
  def paper_trail_enabled_for_controller
    request.user_agent != 'Disable User-Agent'
  end
end

Per class

If you are about change some widgets and you don't want a paper trail of your changes, you can turn PaperTrail off like this:

>> Widget.paper_trail_off

And on again like this:

>> Widget.paper_trail_on

Deleting Old Versions

Over time your versions table will grow to an unwieldy size. Because each version is self-contained (see the Diffing section above for more) you can simply delete any records you don't want any more. For example:

sql> delete from versions where created_at < 2010-06-01;

>> Version.delete_all ["created_at < ?", 1.week.ago]

Installation

Rails 3

1. Install PaperTrail as a gem via your Gemfile:

   gem 'paper_trail', '~> 2'

2. Generate a migration which will add a versions table to your database.

   bundle exec rails generate paper_trail:install

3. Run the migration.

   bundle exec rake db:migrate

4. Add has_paper_trail to the models you want to track.

Rails 2

Please see the rails2 branch.

Testing

PaperTrail uses Bundler to manage its dependencies (in development and testing). You can run the tests with bundle exec rake test. (You may need to bundle install first.)

Articles

Keep a Paper Trail with PaperTrail, Linux Magazine, 16th September 2009.

Problems

Please use GitHub's issue tracker.

Contributors

Many thanks to:

• Zachery Hostens
• Jeremy Weiskotten
• Phan Le
• jdrucza
• conickal
• Thibaud Guillaume-Gentil
• Danny Trelogan
• Mikl Kurkov
• Franco Catena
• Emmanuel Gomez
• Matthew MacLeod
• benzittlau
• Tom Derks
• Jonas Hoglund
• Stefan Huber
• thinkcast
• Dominik Sander
• Burke Libbey
• 6twenty
• nir0

Inspirations

• Simply Versioned
• Acts As Audited

Intellectual Property

Copyright (c) 2011 Andy Stewart (boss@airbladesoftware.com). Released under the MIT licence.