Historiographer

Losing data sucks. Every time you update or destroy a record in Rails, you lose the old data.

Historiographer fixes this problem in a better way than existing auditing gems.

Existing auditing gems for Rails suck

The Audited gem has some serious flaws.

🤚Hands up if your versions table has gotten too big to query 🤚

🤚Hands up if your versions table doesn't have the indexes you need 🤚

🤚Hands up if you've ever iterated over versions records in Ruby to recreate a snapshot of what data looked like at a point in time. 🤚

Why does this happen?

First, audited only tracks a record of what changed, so there's no way to "go back in time" and see what the data looked like back when a problem occurred without replaying every single audit.

Second, it tracks changes as JSON. While some data stores have JSON querying semantics, not all do, making it very hard to ask complex questions of your historical data -- that's the whole reason you're keeping it around.

Third, it doesn't maintain indexes on your data. If you maintain an index on the primary table, wouldn't you also want to look up historical records using the same columns? Historical data is MUCH larger than "latest snapshot" data, so, duh, of course you do.

Finally, Audited creates just one table for all audits. Historical data is big. It's not unusual for an audited gem table to get into the many millions of rows, and need to be constantly partitioned to maintain any kind of efficiency.

How does Historiographer solve these problems?

Historiographer introduces the concept of history tables: append-only tables that have the same structure and indexes as your primary table.

If you have a posts table:

id	title
1	My Great Post
2	My Second Post

You'll also have a post_histories_table:

id	post_id	title	history_started_at	history_ended_at	history_user_id
1	1	My Great Post	'2019-11-08'	NULL	1
2	2	My Second Post	'2019-11-08'	NULL	1

If you change the title of the 1st post:

Post.find(1).update(title: "Title With Better SEO", history_user_id: current_user.id)

You'll expect your posts table to be updated directly:

id	title
1	Title With Better SEO
2	My Second Post

But also, your histories table will be updated:

id	post_id	title	history_started_at	history_ended_at	history_user_id
1	1	My Great Post	'2019-11-08'	'2019-11-09'	1
2	2	My Second Post	'2019-11-08'	NULL	1
1	1	Title With Better SEO	'2019-11-09'	NULL	1

A few things have happened here:

The primary table (posts) is updated directly
The existing history for post_id=1 is timestamped when its history_ended_at, so that we can see when the post had the title "My Great Post"
A new history record is appended to the table containing a complete snapshot of the record, and a NULL history_ended_at. That's because this is the current history.
A record of who made the change is saved (history_user_id). You can join to your users table to see more data.

Getting Started

Whenever you include the Historiographer gem in your ActiveRecord model, it allows you to insert, update, or delete data as you normally would.

class Post < ActiveRecord::Base
  include Historiographer
end

By default, Historiographer will require all SQL-backed methods to provide a history_user_id to track who made the changes.

Post.create(title: "My Post", history_user_id: current_user.id) # => OK
Post.create(title: "My Post") # => Error!

Many existing models will be better off using Historiographer::Safe when getting started, which will not raise an error, but will alert you of locations where your app is missing history_user_id.

class Post < ActiveRecord::Base
  include Historiographer::Safe
end

Create A Migration

You need a separate table to store histories for each model.

So if you have a Posts model:

class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :title, null: false
      t.boolean :enabled
    end
    add_index :posts, :enabled
  end
end

You should create a model named posts_histories:

require "historiographer/postgres_migration"
class CreatePostHistories < ActiveRecord::Migration
  def change
    create_table :post_histories do |t|
      t.histories
    end
  end
end

The t.histories method will automatically create a table with the following columns:

id (because every model has a primary key)
post_id (because this is the foreign key)
title (because it was on the original model)
enabled (because it was on the original model)
history_started_at (to denote when this history became the canonical version)
history_ended_at (to denote when this history was no longer the canonical version, if it has stopped being the canonical version)
history_user_id (to denote the user that made this change, if one is known)

Additionally it will add indices on:

The same columns that had indices on the original model (e.g. enabled)
history_started_at, history_ended_at, and history_user_id

Models

The primary model should include Historiographer:

class Post < ActiveRecord::Base
  include Historiographer
end

You should also make a PostHistory class if you're going to query PostHistory from Rails:

class PostHistory < ActiveRecord::Base
end

The Posts class will acquire a histories method, and the PostHistory model will gain a post method:

p = Post.first
p.histories.first.class

# => "PostHistory"

p.histories.first.post == p
# => true

Creating, Updating, and Destroying Data:

You can just use normal ActiveRecord methods, and all will record histories:

Post.create(title: "My Great Title", history_user_id: current_user.id)
Post.find_by(title: "My Great Title").update(title: "A New Title", history_user_id: current_user.id)
Post.update_all(title: "They're all the same!", history_user_id: current_user.id)
Post.last.destroy!(history_user_id: current_user.id)
Post.destroy_all(history_user_id: current_user.id)

The histories classes have a current method, which only finds current history records. These records will also be the same as the data in the primary table.

p = Post.first
p.current_history

PostHistory.current

What to do when generated index names are too long

Sometimes the generated index names are too long. Just like with standard Rails migrations, you can override the name of the index to fix this problem. To do so, use the index_names argument to override individual index names:

require "historiographer/postgres_migration"
class CreatePostHistories < ActiveRecord::Migration
  def change
    create_table :post_histories do |t|
      t.histories, index_names: {
        title: "my_index_name",
        [:compound, :index] => "my_compound_index_name"
      }
    end
  end
end