ash_archival/documentation/topics/unarchiving.md

Un-archiving

If you want to unarchive a resource that uses a base filter, you will need to define a separate resource that uses the same storage and has no base filter. The rest of this guide applies for folks who aren't using a base_filter.

Un-archiving can be accomplished by creating a read action that is skipped, using exclude_read_actions. Then, you can create an update action that sets that attribute to nil. For example:

archive do
  ...
  exclude_read_actions :archived
end

actions do
  read :archived do
    filter expr(not is_nil(archived_at))
  end

  update :unarchive do
    change set_attribute(:archived_at, nil)
    # if an individual record is used to unarchive
    # it must use the `archived` read action for its atomic upgrade
    atomic_upgrade_with :archived
  end
end

You could then do something like this:

Resource
|> Ash.get!(id, action: :archived)
|> Ash.Changeset.for_update(:unarchive, %{})
|> Ash.update!()

More idiomatically, you would define a code interfaceon the domain, and call that:

# to unarchive by `id`
Resource
|> Ash.Query.for_read(:archived, %{})
|> Ash.Query.filter(id == ^id)
|> Domain.unarchive!()