Record the full lifecycle contract in NodeAction entries

A contract comment above NodeAction.record! now specifies every
verb's metadata shape. NodeAction.head_diff computes the publish
diff between an outgoing head and its replacement -- default-locale
title pair always, author/tags pairs and template/assets/abstract/
body flags only when changed, and a per-locale translation_diff
with added/removed/changed status. It is a pure function of its two
pages, shared by publish, rollback, and the future backfill, and
reads translation rows directly so fallbacks never masquerade as
content.

publish entries carry via ("draft" or "revision"); restore_revision!
is now transactional, takes the acting user, and logs through the
same diff. Staged slug/parent changes applied at publish log a move
entry with the path pair. Node creation logs a create entry with
initial title and path. The draft-scoped translation_destroy writer
is retired -- locale removal is recorded by the publish diff, where
it becomes public fact.

1 files changed, 132 insertions, 0 deletions

diff --git a/app/models/node_action.rb b/app/models/node_action.rb
index f32b5b7..39935c7 100644
--- a/app/models/node_action.rb
+++ b/app/models/node_action.rb
@@ -6,6 +6,82 @@ class NodeAction < ApplicationRecord
   validates :action, presence: true
   validates :occurred_at, presence: true
 
+  # == Metadata contract ==
+  #
+  # metadata is written once at creation and never updated. It is the
+  # single place anything that must survive deletion of the referenced
+  # rows lives; node_id/page_id/user_id are lookup and ordering only.
+  # All keys are strings. Pairs are always {"from" => x, "to" => y} --
+  # never _old/_new suffixes. Optional pairs are written only when the
+  # two sides differ; required keys are always present. All titles and
+  # names are read pinned to I18n.default_locale unless inside a
+  # locale-keyed block.
+  #
+  # Baseline, every entry (written here, not by call sites):
+  #   "username"                 -- actor's login at write time
+  #   "human_readable_node_name" -- node title, default locale
+  #
+  # "create":
+  #   "title" -- initial title, default locale
+  #   "path"  -- unique path at creation time. Historical value only:
+  #              later renames/moves do NOT update old entries. Never
+  #              use as a join key; node_id is the join key while the
+  #              node lives.
+  #
+  # "publish" (any promotion of a page to head; the diff against the
+  # outgoing head is computed BEFORE head is re-pointed, over the
+  # union of both pages' locales, by the shared diff function also
+  # used by the backfill):
+  #   "via"    -- "draft" (ordinary publish) | "revision" (rollback
+  #               via restore_revision!). Always written; absent
+  #               means a pre-contract entry.
+  #   "title"  -- pair, always present. "from" is null on a first
+  #               publish (no outgoing head).
+  #   "author" -- pair, only when the byline (page.user) changed
+  #   "tags"   -- pair of arrays, only when changed
+  #   "assets_changed", "template_changed"
+  #            -- booleans, only when true; page_id links to the
+  #               revision for the real diff (never stored here)
+  #   "abstract_changed", "body_changed"
+  #            -- booleans for the default locale, only when true
+  #   "translation_diff" -- only when any non-default locale differs:
+  #     { "<locale>" => {
+  #         "status"           -- "added" | "removed" | "changed"
+  #         "title"            -- pair; "from" null when added,
+  #                               "to" null when removed
+  #         "abstract_changed", "body_changed" -- booleans, only when
+  #                               true, only for status "changed"
+  #     } }
+  #
+  # "move" (reparenting and/or unique-path change; one entry at the
+  # subtree root, descendants get none -- a descendant's own zoomed
+  # view will not show path history):
+  #   "path"  -- pair
+  #
+  # Reserved for the Trash feature, final shape decided there:
+  # "demote" (via "trash" | "depublish"; carries the leaving-public-
+  # view snapshot: head presence, final published_at), "trash",
+  # "restore_from_trash", "destroy" (final path only; publish-state
+  # snapshot lives on the entry that removed it from public view).
+  # Whether trash logs one entry or a trash+demote pair is decided
+  # with that feature.
+  #
+  # Backfilled entries mirror this vocabulary exactly. Their diff
+  # content is computed, not guessed (consecutive revisions still
+  # exist); only actor (from page.editor) and occurred_at are
+  # inferred, and inferred_from names the heuristic per entry
+  # ("from_page_revision", "from_published_at_heuristic").
+  # inferred_from null = witnessed live.
+  #
+  # The "locale" column is currently written by no verb (it belonged
+  # to the retired translation_destroy) and is retained for backfill
+  # or future draft-scoped verbs.
+  #
+  # This log records; it does not undo. Reversibility stays in
+  # restore_revision! and the revisions system. No IP, session, or
+  # user agent, ever. Success only -- rejected attempts are not
+  # logged.
+
   def self.record!(node:, action:, user: nil, page: nil, locale: nil, **extra)
     create!(
       :node        => node,
@@ -23,6 +99,62 @@ class NodeAction < ApplicationRecord
    )
   end
 
+  # Computes the publish-entry diff between an outgoing head and the
+  # page replacing it, in the exact metadata shape the contract above
+  # specifies. Pure function of its two arguments -- shared verbatim by
+  # publish_draft!, restore_revision!, and the backfill task. Reads
+  # translation rows directly, never locale-dependent accessors, so a
+  # fallback value is never mistaken for real content. Returns
+  # symbol-keyed top level for splatting into record!; nested keys are
+  # strings and jsonb serialization stringifies the rest at write time.
+  def self.head_diff old_page, new_page
+    default = I18n.default_locale
+
+    title_of = ->(page) { page&.translations&.find_by(:locale => default)&.title }
+    diff = { :title => { "from" => title_of.call(old_page),
+                         "to"   => title_of.call(new_page) } }
+    return diff unless old_page
+
+    old_author, new_author = old_page.user&.login, new_page.user&.login
+    diff[:author] = { "from" => old_author, "to" => new_author } if old_author != new_author
+
+    old_tags, new_tags = old_page.tag_list.sort, new_page.tag_list.sort
+    diff[:tags] = { "from" => old_tags, "to" => new_tags } if old_tags != new_tags
+
+    diff[:template_changed] = true if old_page.template_name != new_page.template_name
+    diff[:assets_changed]   = true if old_page.assets.map(&:id) != new_page.assets.map(&:id)
+
+    old_t = old_page.translations.find_by(:locale => default)
+    new_t = new_page.translations.find_by(:locale => default)
+    diff[:abstract_changed] = true if old_t&.abstract != new_t&.abstract
+    diff[:body_changed]     = true if old_t&.body != new_t&.body
+
+    locales = (old_page.translated_locales | new_page.translated_locales) - [default]
+    translation_diff = {}
+
+    locales.sort_by(&:to_s).each do |locale|
+      o = old_page.translations.find_by(:locale => locale)
+      n = new_page.translations.find_by(:locale => locale)
+
+      if o.nil?
+        translation_diff[locale.to_s] = { "status" => "added",
+                                          "title" => { "from" => nil, "to" => n.title } }
+      elsif n.nil?
+        translation_diff[locale.to_s] = { "status" => "removed",
+                                          "title" => { "from" => o.title, "to" => nil } }
+      elsif o.title != n.title || o.abstract != n.abstract || o.body != n.body
+        entry = { "status" => "changed" }
+        entry["title"]            = { "from" => o.title, "to" => n.title } if o.title != n.title
+        entry["abstract_changed"] = true if o.abstract != n.abstract
+        entry["body_changed"]     = true if o.body != n.body
+        translation_diff[locale.to_s] = entry
+      end
+    end
+
+    diff[:translation_diff] = translation_diff if translation_diff.any?
+    diff
+  end
+
   def actor_name
     metadata["username"] || "unknown"
   end