ActiveSnapshot

Simplified snapshots and restoration for ActiveRecord models and associations with a transparent white-box implementation.

Key Features:

• Create and Restore snapshots of a parent record and any specified child records
• Predictable and explicit behaviour provides much needed clarity to your restore logic
• Snapshots are created upon request only, we do not use any callbacks
• Tiny method footprint so its easy to completely override the logic later

Why This Library:

Model Versioning and Restoration require conscious thought, design, and understanding. You should understand your versioning and restoration process completely. This gem's small API and fully understandable design fully supports this.

I do not recommend using paper_trail-association_tracking because it is mostly a blackbox solution which encourages you to set it up and then assume its Just Working^TM. This makes for major data problems later. Dont fall into this trap. Instead read this gems brief source code completely before use OR copy the code straight into your codebase. Once you know it, then you are free.

Installation

gem 'active_snapshot'

Then generate and run the necessary migrations to setup the snapshots and snapshot_items tables.

rails generate active_snapshot:install
rake db:migrate

Then add include ActiveSnapshot to your ApplicationRecord or individual models.

class ApplicationRecord < ActiveRecord::Base
  include ActiveSnapshot
end

This defines the following associations on your models:

has_many :snapshots, as: :item, class_name: 'Snapshot'
has_many :snapshot_items, as: :item, class_name: 'SnapshotItem'

It defines an optional extension to your model: has_snapshot_children.

It defines one instance method to your model: create_snapshot!

Basic Usage

You now have access to the following methods:

post = Post.first

# Create snapshot, all fields are optional
snapshot = post.create_snapshot!(
  identifier: "snapshot_1",
  user: current_user,
  metadata: {
    foo: :bar
  },
)

# Restore snapshot and all its child snapshots
snapshot.restore!

# Destroy snapshot and all its child snapshots
# must be performed manually, snapshots and snapshot items are NEVER destroyed automatically
snapshot.destroy!

Tracking Associated / Child Records

class Post < ActiveRecord::Base
  include ActiveSnapshot

  has_snapshot_children do
    ### Executed in the context of the instance / self

    ### Reload record from database to ensure a clean state and eager load the specified associations
    instance = self.class.includes(:tags, :ip_address, comments: [:comment_sub_records]).find(id)

    ### Define the associated records that will be restored
    {
      comments: instance.comments,

      ### Nested Associations can be handled by simply mapping them into an array
      comment_sub_records: instance.comments.flat_map{|x| x.comment_sub_records },

      tags: {
        records: instance.tags
      },

      ip_address: {
        record: instance.ip_address,
        delete_method: ->(item){ item.release! }
      }
    }
  end

end

Now when you run create_snapshot! the associations will be tracked accordingly

Reifying Snapshots

A reified record refers to an ActiveRecord instance where the local objects data is set to match the snaphotted data, but the database remains changed.

You can view all of the "reified" snapshot items by calling the following method. Its completely up to you on how to use this data.

reified_parent, reified_children_hash = snapshot.fetch_reified_items

As a safety these records have the readonly attribute set on them. If you want to perform any write actions on the returned instances you will have to set the readonly attribute to false

reified_parent, reified_children_hash = snapshot.fetch_reified_items(readonly: false)
# or
reified_parent, reified_children_hash = snapshot.fetch_reified_items
reified_children_hash.first.instance_variable_set("@readonly", false)

Diffing Versions

You can use the following example code to generate your own diffs.

snapshot = post.snapshots.find_by!(identifier: "some-identifier")

snapshot_item = snapshot.snapshot_items.find_by!(item_type: "Post")

old_attrs = snapshot_item.object
new_attrs = post.attributes # or could be another snapshot object

attrs_not_changed = old_attrs.to_a.intersection(new_attrs.to_a).to_h

attrs_changed = new_attrs.to_a - attrs_not_changed.to_a

Important Data Considerations / Warnings

Dropping columns

If you plan to use the snapshot restore capabilities please be aware:

Whenever you drop a database column and there already exists snapshots of that model then you are kind of silently breaking your restore mechanism. Because now the application will not be able to assign data to columns that dont exist on the model. We work around this by bypassing the attribute assignment for snapshot item object entries that does not correlate to a current database column.

I recommend that you add an entry to this in your applications safe-migrations guidelines.

If you would like to detect if this situation has already ocurred you can use the following script:

SnapshotItem.all.each do |snapshot_item|
  snapshot_item.object.keys.each do |key|
    klass = Class.const_get(snapshot_item.item_type)

    if !klass.column_names.include?(key)
      invalid_data = snapshot_item.object.slice(*klass.column_names)

      raise "invalid data found - #{invalid_data}"
    end
  end
end

Key Models Provided & Additional Customizations

A key aspect of this library is its simplicity and small API. For major functionality customizations we encourage you to first delete this gem and then copy this gems code directly into your repository.

I strongly encourage you to read the code for this library to understand how it works within your project so that you are capable of customizing the functionality later.

• SnapshotsConcern
  • Defines snapshots and snapshot_items has_many associations
  • Defines create_snapshot! and has_snapshot_children methods
• Snapshot
  • Contains a unique identifier column (optional, but available for custom identification purposes)
  • has_many :item_snapshots
• SnapshotItem
  • Contains object column which contains an encoded database row
  • belongs_to :snapshot

Credits

Created & Maintained by Weston Ganger - @westonganger