# PaperTrail

[![Build Status][4]][5]
[![Gem Version][53]][54]
[![SemVer][55]][56]

Track changes to your models, for auditing or versioning. See how a model looked
at any stage in its lifecycle, revert it to any version, or restore it after it
has been destroyed.

## Documentation

| Version        | Documentation |
| -------------- | ------------- |
| Unreleased     | https://github.com/paper-trail-gem/paper_trail/blob/master/README.md |
| 12.0.0         | https://github.com/paper-trail-gem/paper_trail/blob/v12.0.0/README.md |
| 11.1.0         | https://github.com/paper-trail-gem/paper_trail/blob/v11.1.0/README.md |
| 10.3.1         | https://github.com/paper-trail-gem/paper_trail/blob/v10.3.1/README.md |
| 9.2.0          | https://github.com/paper-trail-gem/paper_trail/blob/v9.2.0/README.md |
| 8.1.2          | https://github.com/paper-trail-gem/paper_trail/blob/v8.1.2/README.md |
| 7.1.3          | https://github.com/paper-trail-gem/paper_trail/blob/v7.1.3/README.md |
| 6.0.2          | https://github.com/paper-trail-gem/paper_trail/blob/v6.0.2/README.md |
| 5.2.3          | https://github.com/paper-trail-gem/paper_trail/blob/v5.2.3/README.md |
| 4.2.0          | https://github.com/paper-trail-gem/paper_trail/blob/v4.2.0/README.md |
| 3.0.9          | https://github.com/paper-trail-gem/paper_trail/blob/v3.0.9/README.md |
| 2.7.2          | https://github.com/paper-trail-gem/paper_trail/blob/v2.7.2/README.md |
| 1.6.5          | https://github.com/paper-trail-gem/paper_trail/blob/v1.6.5/README.md |

## Table of Contents

<!-- toc -->

- [1. Introduction](#1-introduction)
  - [1.a. Compatibility](#1a-compatibility)
  - [1.b. Installation](#1b-installation)
  - [1.c. Basic Usage](#1c-basic-usage)
  - [1.d. API Summary](#1d-api-summary)
  - [1.e. Configuration](#1e-configuration)
- [2. Limiting What is Versioned, and When](#2-limiting-what-is-versioned-and-when)
  - [2.a. Choosing Lifecycle Events To Monitor](#2a-choosing-lifecycle-events-to-monitor)
  - [2.b. Choosing When To Save New Versions](#2b-choosing-when-to-save-new-versions)
  - [2.c. Choosing Attributes To Monitor](#2c-choosing-attributes-to-monitor)
  - [2.d. Turning PaperTrail Off](#2d-turning-papertrail-off)
  - [2.e. Limiting the Number of Versions Created](#2e-limiting-the-number-of-versions-created)
- [3. Working With Versions](#3-working-with-versions)
  - [3.a. Reverting And Undeleting A Model](#3a-reverting-and-undeleting-a-model)
  - [3.b. Navigating Versions](#3b-navigating-versions)
  - [3.c. Diffing Versions](#3c-diffing-versions)
  - [3.d. Deleting Old Versions](#3d-deleting-old-versions)
- [4. Saving More Information About Versions](#4-saving-more-information-about-versions)
  - [4.a. Finding Out Who Was Responsible For A Change](#4a-finding-out-who-was-responsible-for-a-change)
  - [4.b. Associations](#4b-associations)
  - [4.c. Storing Metadata](#4c-storing-metadata)
- [5. ActiveRecord](#5-activerecord)
  - [5.a. Single Table Inheritance (STI)](#5a-single-table-inheritance-sti)
  - [5.b. Configuring the `versions` Association](#5b-configuring-the-versions-association)
  - [5.c. Generators](#5c-generators)
  - [5.d. Protected Attributes](#5d-protected-attributes)
- [6. Extensibility](#6-extensibility)
  - [6.a. Custom Version Classes](#6a-custom-version-classes)
  - [6.b. Custom Serializer](#6b-custom-serializer)
  - [6.c. Custom Object Changes](#6c-custom-object-changes)
- [7. Testing](#7-testing)
  - [7.a. Minitest](#7a-minitest)
  - [7.b. RSpec](#7b-rspec)
  - [7.c. Cucumber](#7c-cucumber)
  - [7.d. Spork](#7d-spork)
  - [7.e. Zeus or Spring](#7e-zeus-or-spring)
- [8. PaperTrail Plugins](#8-papertrail-plugins)
- [9. Integration with Other Libraries](#9-integration-with-other-libraries)
- [10. Related Libraries and Ports](#10-related-libraries-and-ports)
- [Articles](#articles)
- [Problems](#problems)
- [Contributors](#contributors)
- [Contributing](#contributing)
- [Inspirations](#inspirations)
- [Intellectual Property](#intellectual-property)

<!-- tocstop -->

## 1. Introduction

### 1.a. Compatibility

| paper_trail    | branch     | ruby     | activerecord  |
| -------------- | ---------- | -------- | ------------- |
| unreleased     | master     | >= 2.5.0 | >= 5.2, < 6.2 |
| 12             | master     | >= 2.5.0 | >= 5.2, < 6.2 |
| 11             | master     | >= 2.4.0 | >= 5.2, < 6.1 |
| 10             | 10-stable  | >= 2.3.0 | >= 4.2, < 6.1 |
| 9              | 9-stable   | >= 2.3.0 | >= 4.2, < 5.3 |
| 8              | 8-stable   | >= 2.2.0 | >= 4.2, < 5.2 |
| 7              | 7-stable   | >= 2.1.0 | >= 4.0, < 5.2 |
| 6              | 6-stable   | >= 1.9.3 | >= 4.0, < 5.2 |
| 5              | 5-stable   | >= 1.9.3 | >= 3.0, < 5.1 |
| 4              | 4-stable   | >= 1.8.7 | >= 3.0, < 5.1 |
| 3              | 3.0-stable | >= 1.8.7 | >= 3.0, < 5   |
| 2              | 2.7-stable | >= 1.8.7 | >= 3.0, < 4   |
| 1              | rails2     | >= 1.8.7 | >= 2.3, < 3   |

Experts: to install incompatible versions of activerecord, see
`paper_trail/compatibility.rb`.

### 1.b. Installation

1. Add PaperTrail to your `Gemfile`.

    `gem 'paper_trail'`

1. Add a `versions` table to your database:

    ```
    bundle exec rails generate paper_trail:install [--with-changes]
    ```

    For more information on this generator, see [section 5.c.
    Generators](#5c-generators).

    If using [rails_admin][38], you must enable the
    experimental [Associations](#4b-associations) feature.

    If you're getting "Could not find generator 'paper_trail:install'" errors from
    recent Ruby/Rails versions, try running `spring stop`
    (see [this thread](https://github.com/paper-trail-gem/paper_trail/issues/459) for more details).

    ```
    bundle exec rake db:migrate
    ```

1. Add `has_paper_trail` to the models you want to track.

    ```ruby
    class Widget < ActiveRecord::Base
      has_paper_trail
    end
    ```

1. If your controllers have a `current_user` method, you can easily [track who
is responsible for changes](#4a-finding-out-who-was-responsible-for-a-change)
by adding a controller callback.

    ```ruby
    class ApplicationController
      before_action :set_paper_trail_whodunnit
    end
    ```

### 1.c. Basic Usage

Your models now have a `versions` method which returns the "paper trail" of
changes to your model.

```ruby
widget = Widget.find 42
widget.versions
# [<PaperTrail::Version>, <PaperTrail::Version>, ...]
```

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

```ruby
v = widget.versions.last
v.event # 'update', 'create', 'destroy'. See also: Custom Event Names
v.created_at
v.whodunnit # ID of `current_user`. Requires `set_paper_trail_whodunnit` callback.
widget = v.reify # The widget as it was before the update (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.

```ruby
widget = Widget.find 153
widget.name                                 # 'Doobly'

# Add has_paper_trail to Widget model.

widget.versions                             # []
widget.update name: 'Wotsit'
widget.versions.last.reify.name             # 'Doobly'
widget.versions.last.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*        | *create* | *update* | *destroy* |
| -------------- | -------- | -------- | --------- |
| *Model Before* | nil      | widget   | widget    |
| *Model After*  | widget   | widget   | nil       |

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

### 1.d. API Summary

An introductory sample of common features.

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

```ruby
class Widget < ActiveRecord::Base
  has_paper_trail
end

# Returns this widget's versions.  You can customise the name of the
# association, but overriding this method is not supported.
widget.versions

# Return the version this widget was reified from, or nil if it is live.
# You can customise the name of the method.
widget.version

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

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

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

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

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

And a `PaperTrail::Version` instance (which is just an ordinary ActiveRecord
instance, with all the usual methods) has methods such as:

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

# Return a new item from this version
version.reify(dup: true)

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

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

# 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
```

This is just a sample of common features. Keep reading for more.

### 1.e. Configuration

Many aspects of PaperTrail are configurable for individual models; typically
this is achieved by passing options to the `has_paper_trail` method within
a given model.

Some aspects of PaperTrail are configured globally for all models. These
settings are assigned directly on the `PaperTrail.config` object.
A common place to put these settings is in a Rails initializer file
such as `config/initializers/paper_trail.rb` or in an environment-specific
configuration file such as `config/environments/test.rb`.

#### 1.e.1 Global

Global configuration options affect all threads.

- association_reify_error_behaviour
- enabled
- has_paper_trail_defaults
- object_changes_adapter
- serializer
- version_limit

Syntax example: (options described in detail later)

```ruby
# config/initializers/paper_trail.rb
PaperTrail.config.enabled = true
PaperTrail.config.has_paper_trail_defaults = {
  on: %i[create update destroy]
}
PaperTrail.config.version_limit = 3
````

These options are intended to be set only once, during app initialization (eg.
in `config/initializers`). It is unsafe to change them while the app is running.
In contrast, `PaperTrail.request` has various options that only apply to a
single HTTP request and thus are safe to use while the app is running.

## 2. Limiting What is Versioned, and When

### 2.a. Choosing Lifecycle Events To Monitor

You can choose which events to track with the `on` option.  For example, if
you only want to track `update` events:

```ruby
class Article < ActiveRecord::Base
  has_paper_trail on: [:update]
end
```

`has_paper_trail` installs [callbacks][52] for the specified lifecycle events.

There are four potential callbacks, and the default is to install all four, ie.
`on: [:create, :destroy, :touch, :update]`.

#### The `versions.event` Column

Your `versions` table has an `event` column with three possible values:

| *event* | *callback*    |
| ------- | ------------- |
| create  | create        |
| destroy | destroy       |
| update  | touch, update |

You may also have the `PaperTrail::Version` model save a custom string in its
`event` field instead of the typical `create`, `update`, `destroy`. PaperTrail
adds an `attr_accessor` to your model named `paper_trail_event`, and will insert
it, if present, in the `event` column.

```ruby
a = Article.create
a.versions.size                           # 1
a.versions.last.event                     # 'create'
a.paper_trail_event = 'update title'
a.update title: 'My Title'
a.versions.size                           # 2
a.versions.last.event                     # 'update title'
a.paper_trail_event = nil
a.update title: 'Alternate'
a.versions.size                           # 3
a.versions.last.event                     # 'update'
```

#### Controlling the Order of AR Callbacks

If there are other callbacks in your model, their order relative to those
installed by `has_paper_trail` may matter. If you need to control
their order, use the `paper_trail_on_*` methods.

```ruby
class Article < ActiveRecord::Base
  # Include PaperTrail, but do not install any callbacks. Passing the
  # empty array to `:on` omits callbacks.
  has_paper_trail on: []

  # Add callbacks in the order you need.
  paper_trail.on_destroy    # add destroy callback
  paper_trail.on_update     # etc.
  paper_trail.on_create
  paper_trail.on_touch
end
```

The `paper_trail.on_destroy` method can be further configured to happen
`:before` or `:after` the destroy event. In PaperTrail 4, the default is
`:after`. In PaperTrail 5, the default will be `:before`, to support
ActiveRecord 5. (see https://github.com/paper-trail-gem/paper_trail/pull/683)

### 2.b. Choosing When To Save New Versions

You can choose the conditions when to add new versions with the `if` and
`unless` options. For example, to save versions only for US non-draft
translations:

```ruby
class Translation < ActiveRecord::Base
  has_paper_trail if:     Proc.new { |t| t.language_code == 'US' },
                  unless: Proc.new { |t| t.type == 'DRAFT'       }
end
```

#### Choosing Based on Changed Attributes

Starting with PaperTrail 4.0, versions are saved during an after-callback. If
you decide whether to save a new version based on changed attributes,
use attribute_name_was instead of attribute_name.

#### Saving a New Version Manually

You may want to save a new version regardless of options like `:on`, `:if`, or
`:unless`. Or, in rare situations, you may want to save a new version even if
the record has not changed.

```ruby
my_model.paper_trail.save_with_version
```

### 2.c. Choosing Attributes To Monitor

#### Ignore

You can `ignore` changes to certain attributes:

```ruby
class Article < ActiveRecord::Base
  has_paper_trail ignore: [:title, :rating]
end
```

Changes to just the `title` or `rating` will not create a version record.
Changes to other attributes will create a version record.

```ruby
a = Article.create
a.versions.length                         # 1
a.update title: 'My Title', rating: 3
a.versions.length                         # 1
a.update title: 'Greeting', content: 'Hello'
a.versions.length                         # 2
a.paper_trail.previous_version.title      # 'My Title'
```

The `:ignore` option can also accept `Hash` arguments that we are considering deprecating.

```ruby
class Article < ActiveRecord::Base
  has_paper_trail ignore: [:title, { color: proc { |obj| obj.color == "Yellow" } }]
end
```

#### Only

Or, you can specify a list of the `only` attributes you care about:

```ruby
class Article < ActiveRecord::Base
  has_paper_trail only: [:title]
end
```

Only changes to the `title` will create a version record.

```ruby
a = Article.create
a.versions.length                         # 1
a.update title: 'My Title'
a.versions.length                         # 2
a.update content: 'Hello'
a.versions.length                         # 2
a.paper_trail.previous_version.content    # nil
```

The `:only` option can also accept `Hash` arguments that we are considering deprecating.

```ruby
class Article < ActiveRecord::Base
  has_paper_trail only: [{ title: Proc.new { |obj| !obj.title.blank? } }]
end
```

If the `title` is not blank, then only changes to the `title`
will create a version record.

```ruby
a = Article.create
a.versions.length                         # 1
a.update content: 'Hello'
a.versions.length                         # 2
a.update title: 'Title One'
a.versions.length                         # 3
a.update content: 'Hai'
a.versions.length                         # 3
a.paper_trail.previous_version.content    # "Hello"
a.update title: 'Title Two'
a.versions.length                         # 4
a.paper_trail.previous_version.content    # "Hai"
```

Configuring both `:ignore` and `:only` is not recommended, but it should work as
expected. 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`.

#### Skip

You can skip attributes completely with the `:skip` option.  As with `:ignore`,
updates to these attributes will not create a version record.  In addition, if a
version record is created for some other reason, these attributes will not be
persisted.

```ruby
class Article < ActiveRecord::Base
  has_paper_trail skip: [:file_upload]
end
```

### 2.d. Turning PaperTrail Off

PaperTrail is on by default, but sometimes you don't want to record versions.

#### Per Process

Turn PaperTrail off for **all threads** in a `ruby` process.

```ruby
PaperTrail.enabled = false
```

**Do not use this in production** unless you have a good understanding of
threads vs. processes.

A legitimate use case is to speed up tests. See [Testing](#7-testing) below.

#### Per HTTP Request

```ruby
PaperTrail.request(enabled: false) do
  # no versions created
end
```

or,

```ruby
PaperTrail.request.enabled = false
# no versions created
PaperTrail.request.enabled = true
```

#### Per Class

In the rare case that you need to disable versioning for one model while
keeping versioning enabled for other models, use:

```ruby
PaperTrail.request.disable_model(Banana)
# changes to Banana model do not create versions,
# but eg. changes to Kiwi model do.
PaperTrail.request.enable_model(Banana)
PaperTrail.request.enabled_for_model?(Banana) # => true
```

This setting, as with all `PaperTrail.request` settings, affects only the
current request, not all threads.

For this rare use case, there is no convenient way to pass a block.

##### In a Rails Controller Callback (Not Recommended)

PaperTrail installs a callback in your rails controllers. The installed
callback will call `paper_trail_enabled_for_controller`, which you can
override.

```ruby
class ApplicationController < ActionController::Base
  def paper_trail_enabled_for_controller
    # Don't omit `super` without a good reason.
    super && request.user_agent != 'Disable User-Agent'
  end
end
```

Because you are unable to control the order of callback execution, this
technique is not recommended, but is preserved for backwards compatibility.

It would be better to install your own callback and use
`PaperTrail.request.enabled=` as you see fit.

#### Per Method (Removed)

The `widget.paper_trail.without_versioning` method was removed in v10, without
an exact replacement. To disable versioning, use the [Per Class](#per-class) or
[Per HTTP Request](#per-http-request) methods.

### 2.e. Limiting the Number of Versions Created

Configure `version_limit` to cap the number of versions saved per record. This
does not apply to `create` events.

```ruby
# Limit: 4 versions per record (3 most recent, plus a `create` event)
PaperTrail.config.version_limit = 3
# Remove the limit
PaperTrail.config.version_limit = nil
```

#### 2.e.1 Per-model limit

Models can override the global `PaperTrail.config.version_limit` setting.

Example:

```
# initializer
PaperTrail.config.version_limit = 10

# At most 10 versions
has_paper_trail

# At most 3 versions (2 updates, 1 create). Overrides global version_limit.
has_paper_trail limit: 2

# Infinite versions
has_paper_trail limit: nil
```

To use a per-model limit, your `versions` table must have an
`item_subtype` column. See [Section
4.b.1](https://github.com/paper-trail-gem/paper_trail#4b1-the-optional-item_subtype-column).

## 3. Working With Versions

### 3.a. Reverting And Undeleting A Model

PaperTrail makes reverting to a previous version easy:

```ruby
widget = Widget.find 42
widget.update name: 'Blah blah'
# Time passes....
widget = widget.paper_trail.previous_version  # the widget as it was before the update
widget.save                                   # reverted
```

Alternatively you can find the version at a given time:

```ruby
widget = widget.paper_trail.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:

```ruby
widget = Widget.find(42)
widget.destroy
# Time passes....
widget = Widget.new(id:42)    # creating a new object with the same id, re-establishes the link
versions = widget.versions    # versions ordered by versions.created_at, ascending
widget = versions.last.reify  # the widget as it was before destruction
widget.save                   # the widget lives!
```

You could even use PaperTrail to implement an undo system; [Ryan Bates has!][3]

If your model uses [optimistic locking][1] don't forget to [increment your
`lock_version`][2] before saving or you'll get a `StaleObjectError`.

### 3.b. 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.

```ruby
live_widget = Widget.find 42
live_widget.versions.length                       # 4, for example
widget = live_widget.paper_trail.previous_version # => widget == live_widget.versions.last.reify
widget = widget.paper_trail.previous_version      # => widget == live_widget.versions[-2].reify
widget = widget.paper_trail.next_version          # => widget == live_widget.versions.last.reify
widget.paper_trail.next_version                   # live_widget
```

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

```ruby
widget = Widget.find 42
version = widget.versions[-2]    # assuming widget has several versions
previous_version = version.previous
next_version = version.next
```

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

```ruby
current_version_number = version.index    # 0-based
```

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

```ruby
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?`:

```ruby
widget = Widget.find 42
widget.paper_trail.live?            # true
widget = widget.paper_trail.previous_version
widget.paper_trail.live?            # false
```

And you can perform `WHERE` queries for object versions based on attributes:

```ruby
# Find versions that meet these criteria.
PaperTrail::Version.where_object(content: 'Hello', title: 'Article')

# Find versions before and after attribute `atr` had value `v`:
PaperTrail::Version.where_object_changes(atr: 'v')
```

Using `where_object_changes` to read YAML from a text column was deprecated in
8.1.0, and will now raise an error.

### 3.c. Diffing Versions

There are two scenarios: diffing adjacent versions and diffing non-adjacent
versions.

The best way to diff adjacent versions is to get PaperTrail to do it for you.
If you add an `object_changes` text column to your `versions` table, either at
installation time with the `rails generate paper_trail:install --with-changes`
option or manually, PaperTrail will store the `changes` diff (excluding any
attributes PaperTrail is ignoring) in each `update` version.  You can use the
`version.changeset` method to retrieve it.  For example:

```ruby
widget = Widget.create name: 'Bob'
widget.versions.last.changeset
# {
#   "name"=>[nil, "Bob"],
#   "created_at"=>[nil, 2015-08-10 04:10:40 UTC],
#   "updated_at"=>[nil, 2015-08-10 04:10:40 UTC],
#   "id"=>[nil, 1]
# }
widget.update name: 'Robert'
widget.versions.last.changeset
# {
#   "name"=>["Bob", "Robert"],
#   "updated_at"=>[2015-08-10 04:13:19 UTC, 2015-08-10 04:13:19 UTC]
# }
widget.destroy
widget.versions.last.changeset
# {}
```

Prior to 10.0.0, the `object_changes` were only stored for create and update
events. As of 10.0.0, they are stored for all three events.

Please be aware that PaperTrail doesn't use diffs internally.  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.  This means you can delete any version without
affecting any other.

To diff non-adjacent versions you'll have to write your own code.  These
libraries may help:

For diffing two strings:

* [htmldiff][19]: 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][20]: 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][21]: old-school, line-wise diffs.

For diffing two ActiveRecord objects:

* [Jeremy Weiskotten's PaperTrail fork][22]: uses ActiveSupport's diff to return
  an array of hashes of the changes.
* [activerecord-diff][23]: rather like ActiveRecord::Dirty but also allows you
  to specify which columns to compare.

### 3.d. 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
sql> delete from versions where created_at < '2010-06-01';
```

```ruby
PaperTrail::Version.where('created_at < ?', 1.day.ago).delete_all
```

## 4. Saving More Information About Versions

### 4.a. Finding Out Who Was Responsible For A Change

Set `PaperTrail.request.whodunnit=`, and that value will be stored in the
version's `whodunnit` column.

```ruby
PaperTrail.request.whodunnit = 'Andy Stewart'
widget.update name: 'Wibble'
widget.versions.last.whodunnit # Andy Stewart
```

#### Setting `whodunnit` to a `Proc`

`whodunnit=` also accepts a `Proc`, in the rare case that lazy evaluation is
required.

```ruby
PaperTrail.request.whodunnit = proc do
  caller.find { |c| c.starts_with? Rails.root.to_s }
end
```

Because lazy evaluation can be hard to troubleshoot, this is not
recommended for common use.

#### Setting `whodunnit` Temporarily

To set whodunnit temporarily, for the duration of a block, use
`PaperTrail.request`:

```ruby
PaperTrail.request(whodunnit: 'Dorian Marié') do
  widget.update name: 'Wibble'
end
```

#### Setting `whodunnit` with a controller callback

If your controller has a `current_user` method, PaperTrail provides a
callback that will assign `current_user.id` to `whodunnit`.

```ruby
class ApplicationController
  before_action :set_paper_trail_whodunnit
end
```

You may want `set_paper_trail_whodunnit` 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:

```ruby
class ApplicationController
  def user_for_paper_trail
    logged_in? ? current_member.id : 'Public user'  # or whatever
  end
end
```

See also: [Setting whodunnit in the rails console][33]

#### Terminator and Originator

A version's `whodunnit` column tells us 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` tells us 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.paper_trail_originator`.  And to find out who made a "live" object look
like it does, call `paper_trail_originator` on the object.

```ruby
widget = Widget.find 153                    # assume widget has 0 versions
PaperTrail.request.whodunnit = 'Alice'
widget.update name: 'Yankee'
widget.paper_trail.originator               # 'Alice'
PaperTrail.request.whodunnit = 'Bob'
widget.update name: 'Zulu'
widget.paper_trail.originator               # 'Bob'
first_version, last_version = widget.versions.first, widget.versions.last
first_version.whodunnit                     # 'Alice'
first_version.paper_trail_originator        # nil
first_version.terminator                    # 'Alice'
last_version.whodunnit                      # 'Bob'
last_version.paper_trail_originator         # 'Alice'
last_version.terminator                     # 'Bob'
```

#### Storing an ActiveRecord globalid in whodunnit

If you would like `whodunnit` to return an `ActiveRecord` object instead of a
string, please try the [paper_trail-globalid][37] gem.

### 4.b. Associations

To track and reify associations, use [paper_trail-association_tracking][6] (PT-AT).

From 2014 to 2018, association tracking was an experimental feature, but many
issues were discovered. To attract new volunteers to address these issues, PT-AT
was extracted (see https://github.com/paper-trail-gem/paper_trail/issues/1070).

Even though it had always been an experimental feature, we didn't want the
extraction of PT-AT to be a breaking change, so great care was taken to remove
it slowly.

- In PT 9, PT-AT was kept as a runtime dependency.
- In PT 10, it became a development dependency (If you use it you must add it to
  your own `Gemfile`) and we kept running all of its tests.
- In PT 11, it will no longer be a development dependency, and it is responsible
  for its own tests.

#### 4.b.1 The optional `item_subtype` column

As of PT 10, users may add an `item_subtype` column to their `versions` table.
When storing versions for STI models, rails stores the base class in `item_type`
(that's just how polymorphic associations like `item` work) In addition, PT will
now store the subclass in `item_subtype`. If this column is present PT-AT will
use it to fix a rare issue with reification of STI subclasses.

```ruby
add_column :versions, :item_subtype, :string, null: true
```

So, if you use PT-AT and STI, the addition of this column is recommended.

- https://github.com/paper-trail-gem/paper_trail/issues/594
- https://github.com/paper-trail-gem/paper_trail/pull/1143
- https://github.com/westonganger/paper_trail-association_tracking/pull/5

### 4.c. Storing Metadata

You can add your own custom columns to your `versions` table. Values can be
given using **Model Metadata** or **Controller Metadata**.

#### Model Metadata

You can specify metadata in the model using `has_paper_trail(meta:)`.

```ruby
class Article < ActiveRecord::Base
  belongs_to :author
  has_paper_trail(
    meta: {
      author_id: :author_id, # model attribute
      word_count: :count_words, # arbitrary model method
      answer: 42, # scalar value
      editor: proc { |article| article.editor.full_name } # a Proc
    }
  )
  def count_words
    153
  end
end
```

#### Metadata from Controllers

You can also store any information you like from your controller.  Override
the `info_for_paper_trail` method in your controller to return a hash whose keys
correspond to columns in your `versions` table.

```ruby
class ApplicationController
  def info_for_paper_trail
    { ip: request.remote_ip, user_agent: request.user_agent }
  end
end
```

#### Advantages of Metadata

Why would you do this?  In this example, `author_id` is an attribute of
`Article` and PaperTrail will store it anyway in a serialized 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:

```ruby
PaperTrail::Version.where(author_id: author_id)
```

#### Metadata can Override PaperTrail Columns

**Experts only**. Metadata will override the normal values that PT would have
inserted into its own columns.

| *PT Column*    | *How bad of an idea?* | *Alternative*                 |
| -------------- | --------------------- | ----------------------------- |
| item_type      | terrible idea         |                               |
| item_id        | terrible idea         |                               |
| event          | meh                   | paper_trail_event             |
| whodunnit      | meh                   | PaperTrail.request.whodunnit= |
| object         | a little dangerous    |                               |
| object_changes | a little dangerous    |                               |

## 5. ActiveRecord

### 5.a. Single Table Inheritance (STI)

PaperTrail supports [Single Table Inheritance][39], and even supports an
un-versioned base model, as of `23ffbdc7e1`.

```ruby
class Fruit < ActiveRecord::Base
  # un-versioned base model
end
class Banana < Fruit
  has_paper_trail
end
```

However, there is a known issue when reifying [associations](#associations),
see https://github.com/paper-trail-gem/paper_trail/issues/594

### 5.b. Configuring the `versions` Association

#### 5.b.1. `versions` association

You may configure the name of the `versions` association by passing a different
name (default is `:versions`) in the `versions:` options hash:

```ruby
class Post < ActiveRecord::Base
  has_paper_trail versions: {
    name: :drafts
  }
end

Post.new.versions # => NoMethodError
```

You may pass a
[scope](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many-label-Scopes)
to the `versions` association with the `scope:` option:
```ruby
class Post < ActiveRecord::Base
  has_paper_trail versions: {
    scope: -> { order("id desc") }
  }

  # Equivalent to:
  has_many :versions,
    -> { order("id desc") },
    class_name: 'PaperTrail::Version',
    as: :item
end
```

Any other [options supported by
`has_many`](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many-label-Options)
can be passed along to the `has_many` macro via the `versions:` options hash.

```ruby
class Post < ActiveRecord::Base
  has_paper_trail versions: {
    extend: VersionsExtensions,
    autosave: false
  }
end
```

Overriding (instead of configuring) the `versions` method is not supported.
Overriding associations is not recommended in general.

#### 5.b.2. `item` association

A `PaperTrail::Version` object `belongs_to` an `item`, the relevant record.

The `item` association is first defined in `PaperTrail::VersionConcern`, but
associations can be redefined.

##### Example: adding a `counter_cache` to `item` association

```ruby
# app/models/paper_trail/version.rb
module PaperTrail
  class Version < ActiveRecord::Base
    belongs_to :item, polymorphic: true, counter_cache: true
  end
end
```

When redefining an association, its options are _replaced_ not _merged_, so
don't forget to specify the options from `PaperTrail::VersionConcern`, like
`polymorphic`.

Be advised that redefining an association is an undocumented feature of Rails.

### 5.c. Generators

PaperTrail has one generator, `paper_trail:install`. It writes, but does not
run, a migration file.
The migration adds (at least) the `versions` table. The
most up-to-date documentation for this generator can be found by running `rails
generate paper_trail:install --help`, but a copy is included here for
convenience.

```
Usage:
  rails generate paper_trail:install [options]

Options:
  [--with-changes], [--no-with-changes]            # Store changeset (diff) with each version

Runtime options:
  -f, [--force]                    # Overwrite files that already exist
  -p, [--pretend], [--no-pretend]  # Run but do not make any changes
  -q, [--quiet], [--no-quiet]      # Suppress status output
  -s, [--skip], [--no-skip]        # Skip files that already exist

Generates (but does not run) a migration to add a versions table.
```

### 5.d. Protected Attributes

As of version 6, PT no longer supports rails 3 or the [protected_attributes][17]
gem. If you are still using them, you may use PT 5 or lower. We recommend
upgrading to [strong_parameters][18] as soon as possible.

If you must use [protected_attributes][17] for now, and want to use PT > 5, you
can reopen `PaperTrail::Version` and add the following `attr_accessible` fields:

```ruby
# app/models/paper_trail/version.rb
module PaperTrail
  class Version < ActiveRecord::Base
    include PaperTrail::VersionConcern
    attr_accessible :item_type, :item_id, :event, :whodunnit, :object, :object_changes, :created_at
  end
end
```

This *unsupported workaround* has been tested with protected_attributes 1.0.9 /
rails 4.2.8 / paper_trail 7.0.3.

## 6. Extensibility

### 6.a. Custom Version Classes

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

```ruby
class PostVersion < PaperTrail::Version
  # custom behaviour, e.g:
  self.table_name = :post_versions
end

class Post < ActiveRecord::Base
  has_paper_trail versions: {
    class_name: 'PostVersion'
  }
end
```

Unlike ActiveRecord's `class_name`, you'll have to supply the complete module
path to the class (e.g. `Foo::BarVersion` if your class is inside the module
`Foo`).

#### Advantages

1. For models which have a lot of versions, storing each model's versions in a
   separate table can improve the performance of certain database queries.
1. Store different version [metadata](#4c-storing-metadata) for different models.

#### Configuration

If you are using Postgres, you should also define the sequence that your custom
version class will use:

```ruby
class PostVersion < PaperTrail::Version
  self.table_name = :post_versions
  self.sequence_name = :post_versions_id_seq
end
```

If you only use custom version classes and don't have a `versions` table, you
must let ActiveRecord know that the `PaperTrail::Version` class is an
`abstract_class`.

```ruby
# app/models/paper_trail/version.rb
module PaperTrail
  class Version < ActiveRecord::Base
    include PaperTrail::VersionConcern
    self.abstract_class = true
  end
end
```

You can also specify custom names for the versions and version associations.
This is useful if you already have `versions` or/and `version` methods on your
model.  For example:

```ruby
class Post < ActiveRecord::Base
  has_paper_trail versions: { name: :paper_trail_versions },
                  version:          :paper_trail_version

  # Existing versions method.  We don't want to clash.
  def versions
    # ...
  end

  # Existing version method.  We don't want to clash.
  def version
    # ...
  end
end
```

### 6.b. Custom Serializer

By default, PaperTrail stores your changes as a `YAML` dump. You can override
this with the serializer config option:

```ruby
PaperTrail.serializer = MyCustomSerializer
```

A valid serializer is a `module` (or `class`) that defines a `load` and `dump`
method.  These serializers are included in the gem for your convenience:

* [PaperTrail::Serializers::YAML][24] - Default
* [PaperTrail::Serializers::JSON][25]

#### PostgreSQL JSON column type support

If you use PostgreSQL, and would like to store your `object` (and/or
`object_changes`) data in a column of [type `json` or type `jsonb`][26], specify
`json` instead of `text` for these columns in your migration:

```ruby
create_table :versions do |t|
  # ...
  t.json :object          # Full object changes
  t.json :object_changes  # Optional column-level changes
  # ...
end
```

If you use the PostgreSQL `json` or `jsonb` column type, you do not need
to specify a `PaperTrail.serializer`.

##### Convert existing YAML data to JSON

If you've been using PaperTrail for a while with the default YAML serializer
and you want to switch to JSON or JSONB, you're in a bit of a bind because
there's no automatic way to migrate your data. The first (slow) option is to
loop over every record and parse it in Ruby, then write to a temporary column:

```ruby
add_column :versions, :new_object, :jsonb # or :json
# add_column :versions, :new_object_changes, :jsonb # or :json

# PaperTrail::Version.reset_column_information # needed for rails < 6

PaperTrail::Version.where.not(object: nil).find_each do |version|
  version.update_column(:new_object, YAML.load(version.object))

  # if version.object_changes
  #   version.update_column(
  #     :new_object_changes,
  #     YAML.load(version.object_changes)
  #   )
  # end
end

remove_column :versions, :object
# remove_column :versions, :object_changes
rename_column :versions, :new_object, :object
# rename_column :versions, :new_object_changes, :object_changes
```

This technique can be very slow if you have a lot of data. Though slow, it is
safe in databases where transactions are protected against DDL, such as
Postgres. In databases without such protection, such as MySQL, a table lock may
be necessary.

If the above technique is too slow for your needs, and you're okay doing without
PaperTrail data temporarily, you can create the new column without converting
the data.

```ruby
rename_column :versions, :object, :old_object
add_column :versions, :object, :jsonb # or :json
```

After that migration, your historical data still exists as YAML, and new data
will be stored as JSON. Next, convert records from YAML to JSON using a
background script.

```ruby
PaperTrail::Version.where.not(old_object: nil).find_each do |version|
  version.update_columns old_object: nil, object: YAML.load(version.old_object)
end
```

Finally, in another migration, remove the old column.

```ruby
remove_column :versions, :old_object
```

If you use the optional `object_changes` column, don't forget to convert it
also, using the same technique.

##### Convert a Column from Text to JSON

If your `object` column already contains JSON data, and you want to change its
data type to `json` or `jsonb`, you can use the following [DDL][36]. Of course,
if your `object` column contains YAML, you must first convert the data to JSON
(see above) before you can change the column type.

Using SQL:

```sql
alter table versions
alter column object type jsonb
using object::jsonb;
```

Using ActiveRecord:

```ruby
class ConvertVersionsObjectToJson < ActiveRecord::Migration
  def up
    change_column :versions, :object, 'jsonb USING object::jsonb'
  end

  def down
    change_column :versions, :object, 'text USING object::text'
  end
end
```

### 6.c. Custom Object Changes

To fully control the contents of their `object_changes` column, expert users
can write an adapter.

```ruby
PaperTrail.config.object_changes_adapter = MyObjectChangesAdapter.new

class MyObjectChangesAdapter
  # @param changes Hash
  # @return Hash
  def diff(changes)
    # ...
  end
end
```

You should only use this feature if you are comfortable reading PT's source to
see exactly how the adapter is used. For example, see how `diff` is used by
reading `::PaperTrail::Events::Base#recordable_object_changes`.

An adapter can implement any or all of the following methods:

1. diff: Returns the changeset in the desired format given the changeset in the original format
2. load_changeset: Returns the changeset for a given version object
3. where_object_changes: Returns the records resulting from the given hash of attributes.
4. where_object_changes_from: Returns the records resulting from the given hash of attributes where the attributes changed *from* the provided value(s).

Depending on what your adapter does, you may have to implement all three.

For an example of a complete and useful adapter, see
[paper_trail-hashdiff](https://github.com/hashwin/paper_trail-hashdiff)

### 6.d. Excluding the Object Column

The `object` column ends up storing a lot of duplicate data if you have models that have many columns,
and that are updated many times. You can save ~50% of storage space by removing the column from the
versions table. It's important to note that this will disable `reify` and `where_object`.

## 7. Testing

You may want to turn PaperTrail off to speed up your tests.  See [Turning
PaperTrail Off](#2d-turning-papertrail-off) above.

### 7.a. Minitest

First, disable PT for the entire `ruby` process.

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

Then, to enable PT for specific tests, you can add a `with_versioning` test
helper method.

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

Then, use the helper in your tests.

```ruby
test 'something that needs versioning' do
  with_versioning do
    # your test
  end
end
```

### 7.b. RSpec

PaperTrail provides a helper, `paper_trail/frameworks/rspec.rb`, that works with
[RSpec][27] to make it easier to control when `PaperTrail` is enabled during
testing.

```ruby
# spec/rails_helper.rb
ENV["RAILS_ENV"] ||= 'test'
require 'spec_helper'
require File.expand_path("../../config/environment", __FILE__)
require 'rspec/rails'
# ...
require 'paper_trail/frameworks/rspec'
```

With the helper loaded, PaperTrail will be turned off for all tests by
default. To enable PaperTrail for a test you can either wrap the
test in a `with_versioning` block, or pass in `versioning: true` option to a
spec block.

```ruby
describe 'RSpec test group' do
  it 'by default, PaperTrail will be turned off' do
    expect(PaperTrail).to_not be_enabled
  end

  with_versioning do
    it 'within a `with_versioning` block it will be turned on' do
      expect(PaperTrail).to be_enabled
    end
  end

  it 'can be turned on at the `it` or `describe` level', versioning: true do
    expect(PaperTrail).to be_enabled
  end
end
```

The helper will also reset `whodunnit` to `nil` before each
test to help prevent data spillover between tests. If you are using PaperTrail
with Rails, the helper will automatically set the
`PaperTrail.request.controller_info` value to `{}` as well, again, to help
prevent data spillover between tests.

There is also a `be_versioned` matcher provided by PaperTrail's RSpec helper
which can be leveraged like so:

```ruby
class Widget < ActiveRecord::Base
end

describe Widget do
  it 'is not versioned by default' do
    is_expected.to_not be_versioned
  end

  describe 'add versioning to the `Widget` class' do
    before(:all) do
      class Widget < ActiveRecord::Base
        has_paper_trail
      end
    end

    it 'enables paper trail' do
      is_expected.to be_versioned
    end
  end
end
```

#### Matchers

The `have_a_version_with` matcher makes assertions about versions using
`where_object`, based on the `object` column.

```ruby
describe '`have_a_version_with` matcher' do
  it 'is possible to do assertions on version attributes' do
    widget.update!(name: 'Leonard', an_integer: 1)
    widget.update!(name: 'Tom')
    widget.update!(name: 'Bob')
    expect(widget).to have_a_version_with name: 'Leonard', an_integer: 1
    expect(widget).to have_a_version_with an_integer: 1
    expect(widget).to have_a_version_with name: 'Tom'
  end
end
```

The `have_a_version_with_changes` matcher makes assertions about versions using
`where_object_changes`, based on the optional
[`object_changes` column](#3c-diffing-versions).

```ruby
describe '`have_a_version_with_changes` matcher' do
  it 'is possible to do assertions on version changes' do
    widget.update!(name: 'Leonard', an_integer: 1)
    widget.update!(name: 'Tom')
    widget.update!(name: 'Bob')
    expect(widget).to have_a_version_with_changes name: 'Leonard', an_integer: 2
    expect(widget).to have_a_version_with_changes an_integer: 2
    expect(widget).to have_a_version_with_changes name: 'Bob'
  end
end
```

For more examples of the RSpec matchers, see the
[Widget spec](https://github.com/paper-trail-gem/paper_trail/blob/master/spec/models/widget_spec.rb)

### 7.c. Cucumber

PaperTrail provides a helper for [Cucumber][28] that works similar to the RSpec
helper. If you want to use the helper, you will need to require in your cucumber
helper like so:

```ruby
# features/support/env.rb

ENV["RAILS_ENV"] ||= 'cucumber'
require File.expand_path(File.dirname(__FILE__) + '/../../config/environment')
# ...
require 'paper_trail/frameworks/cucumber'
```

When the helper is loaded, PaperTrail will be turned off for all scenarios by a
`before` hook added by the helper by default. When you want to enable PaperTrail
for a scenario, you can wrap code in a `with_versioning` block in a step, like
so:

```ruby
Given /I want versioning on my model/ do
  with_versioning do
    # PaperTrail will be turned on for all code inside of this block
  end
end
```

The helper will also reset the `whodunnit` value to `nil` before each
test to help prevent data spillover between tests. If you are using PaperTrail
with Rails, the helper will automatically set the
`PaperTrail.request.controller_info` value to `{}` as well, again, to help
prevent data spillover between tests.

### 7.d. Spork

If you want to use the `RSpec` or `Cucumber` helpers with [Spork][29], you will
need to manually require the helper(s) in your `prefork` block on your test
helper, like so:

```ruby
# spec/rails_helper.rb

require 'spork'

Spork.prefork do
  # This file is copied to spec/ when you run 'rails generate rspec:install'
  ENV["RAILS_ENV"] ||= 'test'
  require 'spec_helper'
  require File.expand_path("../../config/environment", __FILE__)
  require 'rspec/rails'
  require 'paper_trail/frameworks/rspec'
  require 'paper_trail/frameworks/cucumber'
  # ...
end
```

### 7.e. Zeus or Spring

If you want to use the `RSpec` or `Cucumber` helpers with [Zeus][30] or
[Spring][31], you will need to manually require the helper(s) in your test
helper, like so:

```ruby
# spec/rails_helper.rb

ENV["RAILS_ENV"] ||= 'test'
require 'spec_helper'
require File.expand_path("../../config/environment", __FILE__)
require 'rspec/rails'
require 'paper_trail/frameworks/rspec'
```

## 8. PaperTrail Plugins
- [paper_trail-association_tracking][6] - track and reify associations
- [paper_trail-globalid][49] - enhances whodunnit by adding an `actor`

## 9. Integration with Other Libraries

- [ActiveAdmin][42]
- [paper_trail_manager][46] - Browse, subscribe, view and revert changes to
  records with rails and paper_trail
- [rails_admin_history_rollback][51] - History rollback for rails_admin with PT
- Sinatra - [paper_trail-sinatra][41]
- [globalize][45] - [globalize-versioning][44]
- [solidus_papertrail][47] - PT integration for Solidus
  method to instances of PaperTrail::Version that returns the ActiveRecord
  object who was responsible for change

## 10. Related Libraries and Ports

- [izelnakri/paper_trail][50] - An Ecto library, inspired by PT.
- [sequelize-paper-trail][48] - A JS library, inspired by PT. A sequelize
  plugin for tracking revision history of model instances.

## Articles

* [PaperTrail Gem Tutorial](https://stevepolito.design/blog/paper-trail-gem-tutorial/), 20th April 2020.
* [Jutsu #8 - Version your RoR models with PaperTrail](http://samurails.com/gems/papertrail/),
  [Thibault](http://samurails.com/about-me/), 29th September 2014
* [Versioning with PaperTrail](http://www.sitepoint.com/versioning-papertrail),
  [Ilya Bodrov](http://www.sitepoint.com/author/ibodrov), 10th April 2014
* [Using PaperTrail to track stack traces](http://web.archive.org/web/20141120233916/http://rubyrailsexpert.com/?p=36),
  T James Corcoran's blog, 1st October 2013.
* [RailsCast #255 - Undo with PaperTrail](http://railscasts.com/episodes/255-undo-with-paper-trail),
  28th February 2011.
* [Keep a Paper Trail with PaperTrail](http://www.linux-mag.com/id/7528),
  Linux Magazine, 16th September 2009.

## Problems

Please use GitHub's [issue tracker](https://github.com/paper-trail-gem/paper_trail/issues).

## Contributors

Created by Andy Stewart in 2010, maintained since 2012 by Ben Atkins, since 2015
by Jared Beck, with contributions by over 150 people.

https://github.com/paper-trail-gem/paper_trail/graphs/contributors

## Contributing

See our [contribution guidelines][43]

## Inspirations

* [Simply Versioned](https://github.com/jerome/simply_versioned)
* [Acts As Audited](https://github.com/collectiveidea/audited)

## Intellectual Property

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

[1]: http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html
[2]: https://github.com/paper-trail-gem/paper_trail/issues/163
[3]: http://railscasts.com/episodes/255-undo-with-paper-trail
[4]: https://api.travis-ci.org/paper-trail-gem/paper_trail.svg?branch=master
[5]: https://travis-ci.org/paper-trail-gem/paper_trail
[6]: https://github.com/westonganger/paper_trail-association_tracking
[9]: https://github.com/paper-trail-gem/paper_trail/tree/3.0-stable
[10]: https://github.com/paper-trail-gem/paper_trail/tree/2.7-stable
[11]: https://github.com/paper-trail-gem/paper_trail/tree/rails2
[14]: https://raw.github.com/paper-trail-gem/paper_trail/master/lib/generators/paper_trail/templates/create_versions.rb
[16]: https://github.com/paper-trail-gem/paper_trail/issues/113
[17]: https://github.com/rails/protected_attributes
[18]: https://github.com/rails/strong_parameters
[19]: http://github.com/myobie/htmldiff
[20]: http://github.com/pvande/differ
[21]: https://github.com/halostatue/diff-lcs
[22]: http://github.com/jeremyw/paper_trail/blob/master/lib/paper_trail/has_paper_trail.rb#L151-156
[23]: http://github.com/tim/activerecord-diff
[24]: https://github.com/paper-trail-gem/paper_trail/blob/master/lib/paper_trail/serializers/yaml.rb
[25]: https://github.com/paper-trail-gem/paper_trail/blob/master/lib/paper_trail/serializers/json.rb
[26]: http://www.postgresql.org/docs/9.4/static/datatype-json.html
[27]: https://github.com/rspec/rspec
[28]: http://cukes.info
[29]: https://github.com/sporkrb/spork
[30]: https://github.com/burke/zeus
[31]: https://github.com/rails/spring
[32]: http://api.rubyonrails.org/classes/ActiveRecord/AutosaveAssociation.html#method-i-mark_for_destruction
[33]: https://github.com/paper-trail-gem/paper_trail/wiki/Setting-whodunnit-in-the-rails-console
[34]: https://github.com/rails/rails/blob/591a0bb87fff7583e01156696fbbf929d48d3e54/activerecord/lib/active_record/fixtures.rb#L142
[35]: https://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html
[36]: http://www.postgresql.org/docs/9.4/interactive/ddl.html
[37]: https://github.com/ankit1910/paper_trail-globalid
[38]: https://github.com/sferik/rails_admin
[39]: http://api.rubyonrails.org/classes/ActiveRecord/Base.html#class-ActiveRecord::Base-label-Single+table+inheritance
[40]: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#module-ActiveRecord::Associations::ClassMethods-label-Polymorphic+Associations
[41]: https://github.com/jaredbeck/paper_trail-sinatra
[42]: https://github.com/activeadmin/activeadmin/wiki/Auditing-via-paper_trail-%28change-history%29
[43]: https://github.com/paper-trail-gem/paper_trail/blob/master/.github/CONTRIBUTING.md
[44]: https://github.com/globalize/globalize-versioning
[45]: https://github.com/globalize/globalize
[46]: https://github.com/fusion94/paper_trail_manager
[47]: https://github.com/solidusio-contrib/solidus_papertrail
[48]: https://github.com/nielsgl/sequelize-paper-trail
[49]: https://github.com/ankit1910/paper_trail-globalid
[50]: https://github.com/izelnakri/paper_trail
[51]: https://github.com/rikkipitt/rails_admin_history_rollback
[52]: http://guides.rubyonrails.org/active_record_callbacks.html
[53]: https://badge.fury.io/rb/paper_trail.svg
[54]: https://rubygems.org/gems/paper_trail
[55]: https://api.dependabot.com/badges/compatibility_score?dependency-name=paper_trail&package-manager=bundler&version-scheme=semver
[56]: https://dependabot.com/compatibility-score.html?dependency-name=paper_trail&package-manager=bundler&version-scheme=semver