getting-started-with-ash-paper-trail.md

Getting started with AshPaperTrail

First, add the dependency to your mix.exs

{:ash_paper_trail, "~> 0.5.5"}

Next add :ash_ paper_trail to your .formatter.exs under import_deps.

  [
    import_deps: [
      :ash_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,
  domain: MyDomain,
  extensions: [
    AshPaperTrail.Resource
  ]

  paper_trail do
    primary_key_type :uuid_v7 # default is :uuid
    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
    ignore_actions [:destroy] # default is []
  end

This will generate the version resource automatically and add them to your domain. 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

Including version resources in your domain

First, add the AshPaperTrail.Domain extension to your domain.

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

Including all version resources:

Set include_versions? true in the configuration, like so:

paper_trail do
  include_versions? true
end

Including specific version resources

Alternatively, you can configure individual version resources, like so:

resources do
  resource MyApp.Post
  resource MyApp.Post.Version # <- add version resource
end

Destroy Actions

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

1. (preferred) use the version mixin to set on_delete: :delete_all on the ference.

paper_trail do
  mixin {MyApp.MyResource.PaperTrailMixin, :mixin, []}
end

...

defmodule MyApp.MyResource.PaperTrailMixin do
  def mixin do
    # quote here is because we are returning code to be evaluated inside of the
    # calling module
    quote do
      postgres do
        reference :version_source, on_delete: :delete_all
      end
    end
  end
end

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

3. configure AshPaperTrail to ignore the reference, via:

paper_trail do
  reference_source? false
end

and you could then use the mixin described below to add on_delete options to the reference.

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, domain: MyApp.Accounts
  belongs_to_actor :news_feed, MyApp.Accounts.NewsFeed, domain: 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, :graphql, [:my_resource_version]}
  relationship_opts public?: true
  version_extensions extensions: [AshGraphql.Resource]
end

And then you can define a module like so:

defmodule MyApp.MyResource.PaperTrailMixin do

  def graphql(type) do
    quote do
      graphql do
        type unquote(type)

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

Tracking Changes Conditionally

By default changes are tracked for all actions that "change" something. You can change this by setting the only_when_changed? option to true in the paper_trail DSL.

You may want to opt into this conditionally. To do this, set the context %{skip_version_when_unchanged?: true}. For example, one specific action might have a change like this:

change set_context(%{skip_version_when_unchanged?: true})

to instruct AshPaperTrail not to create a version if there are no changed for this particular action.