Settings View Source Get started with paper trail

Then, add the AshPaperTrail.Resource extension to any resource you would like to version and configure the change tracking mode

use Ash.Resource,
  extensions: [
    AshPaperTrail.Resource
  ]

  paper_trail do
    change_tracking_mode :changes_only # default is :snapshot
    store_action_name? true # default is false
    ignore_attributes [:inserted_at, :updated_at] # the primary keys are always ignored
  end

This will generate the version resource automatically and add them to your api. The autogenerated resource will be named Version under the namespace of the original resource and will belong to the original resource. For example, if your original resource is MyApp.Post the autogenerated resource will be MyApp.Post.Version. Post has_many paper_trail_versions and Version belong_to source_version

Then for each Api for versioned resources, you need the versioned resource to the Api of the original non-versioned resource's api. There are a few ways to achieve this.

If your Api doesn't use a registry and you're declaring your resources directly in your Api (this is strongly preferred), add the AshPaperTrail.Api extension to api.

use Ash.Api,
  extensions: [
    AshPaperTrail.Api
  ]

This approach does not add the resources to the api, but does allow them to be accessed via the api. In most use cases, this is acceptable; however, in some use cases you'll need each resource to be a part of the api--if your versioned resources are part of a graphql. If that's the case, add each versioned resource explicitly to your api's resources.

use Ash.Api

resources do
  resource MyApp.Post
  resource MyApp.Post.Version
end

If the Api uses a registry, consider removing the registry and going with one the options above. This third option is deprecated and only for backwards compatibility. Add the AshPaperTrail.Registry extension to the registry.

use Ash.Registry,
  extensions: [
    AshPaperTrail.Registry
  ]

Destroy Actions

If you are using AshPostgres, and you want to support destroy actions, you need to do one of two things:

1. use something like AshArchival in conjunction with this resource to ensure that destroy actions are soft? and do not actually result in row deletion

2. configure AshPaperTrail not to create references, via:

paper_trail do
  reference_source? false
end

Attributes

By default, attribute values are stored in the changes attribute. This is to protect you over time as your resources change. However, if there are attributes that you are confident will not change, you can create attributes for them on the version resource, like so:

paper_trail do
  attributes_as_attributes [:organization_id, :author_id]
end

This will make your version resource have foo and bar attributes (they will still show up in changes), i.e

%ThingVersion{foo: "foo", bar: "bar", changes: %{"foo" => "foo", "bar" => "bar"}}

Change Tracking Modes

Valid options are :snapshot and :changes_only and :full_diff.

Snapshots

:snapshot will json dump the contents of every attribute whether they changed or not.

{ subject: "new subject", body: "unchanged body", author: { name: "bob"}}

Changes Only

:changes_only will json dump the contents of only the attributes that have changed. Note if any part of an embedded attribute and array of embedded attributes, changes then the entire top level attribute is dumped.

{ subject: "new subject" }

Full Diff

:full_diff will json dump the contents of each attribute.

{ subject: { from: "subject", to: "new subject" }, body: { unchanged: "unchanged_body" }}, author: { changes: { unchanged: "bob" }}

Associating Versions with Actors

You can record the actor who made the change by declaring one or more resources that can be actors.

paper_trail do
  belongs_to_actor :user, MyApp.Accounts.User, api: MyApp.Accounts
  belongs_to_actor :news_feed, MyApp.Accounts.NewsFeed, api: MyApp.Accounts
end

Each belongs_to_actor will create a belongs_to relationship with the given name destination. When creating a new version, if the actor on the action is set and matches the resource type, the version will be related to the actor. If your actors are polymorphic or varying types, declare a belongs_to_actor for each type.

A reference is also created with on_delete: :nilify and on_update: :update

If you need a more complex relationship or your actor is not a resource (e.g. String), the actor is always set on Version create and you can store it by adding :on_create change in a mixin.

Multitenancy

If your resource uses multitenancy, then the strategy, attribute, and parse_attribute options (if any) will be applied to the version resource. If using the attribute strategy you will need to ensure this is also an attribute on the version using the attributes_as_attributes option (described above) or via a mixin (described below)

Enriching the Versions resource

If you want to do something like exposing your versions resource over your graphql, you can use the mixin and version_extensions options.

For example:

paper_trail do
  mixin MyApp.MyResource.PaperTrailMixin
  version_extensions extensions: [AshGraphql.Resource]
end

And then you can define a module like so:

defmodule MyApp.MyResource.PaperTrailMixin do

  defmacro __using__(_) do
    quote do
      graphql do
        type :my_resource_version

        queries do
          list :list_versions, action: :read
        end
      end
    end
  end
end