From 65b4a0a6f1c19adde71df567dd7a9f0f6af5dc14 Mon Sep 17 00:00:00 2001
From: Jesse Karmani <jessekarmani@gmail.com>
Date: Fri, 5 Sep 2025 12:28:29 -0700
Subject: [PATCH] Implement FEP 7888: Part 1 - publish conversation context
 (#35959)

---
 .../activitypub/contexts_controller.rb        |  81 +++++++++++
 app/lib/activitypub/tag_manager.rb            |   8 ++
 app/models/conversation.rb                    |  27 +++-
 app/models/status.rb                          |   5 +-
 .../activitypub/context_presenter.rb          |  14 ++
 .../activitypub/context_serializer.rb         |  11 ++
 .../activitypub/note_serializer.rb            |   8 +-
 config/routes.rb                              |   5 +
 ...tus_and_parent_account_to_conversations.rb |   8 ++
 ...0_add_index_on_conversation_to_statuses.rb |   9 ++
 db/schema.rb                                  |   5 +-
 spec/fabricators/conversation_fabricator.rb   |   4 +-
 spec/models/account_conversation_spec.rb      |   8 +-
 spec/requests/activitypub/contexts_spec.rb    | 127 ++++++++++++++++++
 .../activitypub/note_serializer_spec.rb       |   1 +
 15 files changed, 309 insertions(+), 12 deletions(-)
 create mode 100644 app/controllers/activitypub/contexts_controller.rb
 create mode 100644 app/presenters/activitypub/context_presenter.rb
 create mode 100644 app/serializers/activitypub/context_serializer.rb
 create mode 100644 db/migrate/20250828222741_add_parent_status_and_parent_account_to_conversations.rb
 create mode 100644 db/migrate/20250902221600_add_index_on_conversation_to_statuses.rb
 create mode 100644 spec/requests/activitypub/contexts_spec.rb

diff --git a/app/controllers/activitypub/contexts_controller.rb b/app/controllers/activitypub/contexts_controller.rb
new file mode 100644
index 0000000000..2a7e69cff9
--- /dev/null
+++ b/app/controllers/activitypub/contexts_controller.rb
@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+class ActivityPub::ContextsController < ActivityPub::BaseController
+  vary_by -> { 'Signature' if authorized_fetch_mode? }
+
+  before_action :require_account_signature!, if: :authorized_fetch_mode?
+  before_action :set_conversation
+  before_action :set_items
+
+  DESCENDANTS_LIMIT = 60
+
+  def show
+    expires_in 3.minutes, public: public_fetch_mode?
+    render_with_cache json: context_presenter, serializer: ActivityPub::ContextSerializer, adapter: ActivityPub::Adapter, content_type: 'application/activity+json'
+  end
+
+  def items
+    expires_in 3.minutes, public: public_fetch_mode?
+    render_with_cache json: items_collection_presenter, serializer: ActivityPub::CollectionSerializer, adapter: ActivityPub::Adapter, content_type: 'application/activity+json'
+  end
+
+  private
+
+  def account_required?
+    false
+  end
+
+  def set_conversation
+    @conversation = Conversation.local.find(params[:id])
+  end
+
+  def set_items
+    @items = @conversation.statuses.distributable_visibility.paginate_by_min_id(DESCENDANTS_LIMIT, params[:min_id])
+  end
+
+  def context_presenter
+    first_page = ActivityPub::CollectionPresenter.new(
+      id: items_context_url(@conversation, page_params),
+      type: :unordered,
+      part_of: items_context_url(@conversation),
+      next: next_page,
+      items: @items.map { |status| status.local? ? ActivityPub::TagManager.instance.uri_for(status) : status.uri }
+    )
+
+    ActivityPub::ContextPresenter.from_conversation(@conversation).tap do |presenter|
+      presenter.first = first_page
+    end
+  end
+
+  def items_collection_presenter
+    page = ActivityPub::CollectionPresenter.new(
+      id: items_context_url(@conversation, page_params),
+      type: :unordered,
+      part_of: items_context_url(@conversation),
+      next: next_page,
+      items: @items.map { |status| status.local? ? ActivityPub::TagManager.instance.uri_for(status) : status.uri }
+    )
+
+    return page if page_requested?
+
+    ActivityPub::CollectionPresenter.new(
+      id: items_context_url(@conversation),
+      type: :unordered,
+      first: page
+    )
+  end
+
+  def page_requested?
+    truthy_param?(:page)
+  end
+
+  def next_page
+    return nil if @items.size < DESCENDANTS_LIMIT
+
+    items_context_url(@conversation, page: true, min_id: @items.last.id)
+  end
+
+  def page_params
+    params.permit(:page, :min_id)
+  end
+end
diff --git a/app/lib/activitypub/tag_manager.rb b/app/lib/activitypub/tag_manager.rb
index 975763e82f..9f72a150d3 100644
--- a/app/lib/activitypub/tag_manager.rb
+++ b/app/lib/activitypub/tag_manager.rb
@@ -40,6 +40,8 @@ class ActivityPub::TagManager
     case target.object_type
     when :person
       target.instance_actor? ? instance_actor_url : account_url(target)
+    when :conversation
+      context_url(target)
     when :note, :comment, :activity
       return activity_account_status_url(target.account, target) if target.reblog?
 
@@ -76,6 +78,12 @@ class ActivityPub::TagManager
     activity_account_status_url(target.account, target)
   end
 
+  def context_uri_for(target, page_params = nil)
+    raise ArgumentError, 'target must be a local activity' unless %i(note comment activity).include?(target.object_type) && target.local?
+
+    items_context_url(target.conversation, page_params)
+  end
+
   def replies_uri_for(target, page_params = nil)
     raise ArgumentError, 'target must be a local activity' unless %i(note comment activity).include?(target.object_type) && target.local?
 
diff --git a/app/models/conversation.rb b/app/models/conversation.rb
index a7fe148840..c86b90ad7d 100644
--- a/app/models/conversation.rb
+++ b/app/models/conversation.rb
@@ -4,10 +4,12 @@
 #
 # Table name: conversations
 #
-#  id         :bigint(8)        not null, primary key
-#  uri        :string
-#  created_at :datetime         not null
-#  updated_at :datetime         not null
+#  id                :bigint(8)        not null, primary key
+#  uri               :string
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#  parent_account_id :bigint(8)
+#  parent_status_id  :bigint(8)
 #
 
 class Conversation < ApplicationRecord
@@ -15,7 +17,24 @@ class Conversation < ApplicationRecord
 
   has_many :statuses, dependent: nil
 
+  belongs_to :parent_status, class_name: 'Status', optional: true, inverse_of: :conversation
+  belongs_to :parent_account, class_name: 'Account', optional: true
+
+  scope :local, -> { where(uri: nil) }
+
+  before_validation :set_parent_account, on: :create
+
   def local?
     uri.nil?
   end
+
+  def object_type
+    :conversation
+  end
+
+  private
+
+  def set_parent_account
+    self.parent_account = parent_status.account if parent_status.present?
+  end
 end
diff --git a/app/models/status.rb b/app/models/status.rb
index e933c92cae..f72a92078b 100644
--- a/app/models/status.rb
+++ b/app/models/status.rb
@@ -70,6 +70,8 @@ class Status < ApplicationRecord
     belongs_to :reblog, foreign_key: 'reblog_of_id', inverse_of: :reblogs
   end
 
+  has_one :owned_conversation, class_name: 'Conversation', foreign_key: 'parent_status_id', inverse_of: :parent_status, dependent: nil
+
   has_many :favourites, inverse_of: :status, dependent: :destroy
   has_many :bookmarks, inverse_of: :status, dependent: :destroy
   has_many :reblogs, foreign_key: 'reblog_of_id', class_name: 'Status', inverse_of: :reblog, dependent: :destroy
@@ -442,7 +444,8 @@ class Status < ApplicationRecord
       self.in_reply_to_account_id = carried_over_reply_to_account_id
       self.conversation_id        = thread.conversation_id if conversation_id.nil?
     elsif conversation_id.nil?
-      self.conversation = Conversation.new
+      conversation = build_owned_conversation
+      self.conversation = conversation
     end
   end
 
diff --git a/app/presenters/activitypub/context_presenter.rb b/app/presenters/activitypub/context_presenter.rb
new file mode 100644
index 0000000000..c13900ffda
--- /dev/null
+++ b/app/presenters/activitypub/context_presenter.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class ActivityPub::ContextPresenter < ActiveModelSerializers::Model
+  attributes :id, :type, :attributed_to, :first, :object_type
+
+  class << self
+    def from_conversation(conversation)
+      new.tap do |presenter|
+        presenter.id = ActivityPub::TagManager.instance.uri_for(conversation)
+        presenter.attributed_to = ActivityPub::TagManager.instance.uri_for(conversation.parent_account)
+      end
+    end
+  end
+end
diff --git a/app/serializers/activitypub/context_serializer.rb b/app/serializers/activitypub/context_serializer.rb
new file mode 100644
index 0000000000..33c70d8e9d
--- /dev/null
+++ b/app/serializers/activitypub/context_serializer.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ActivityPub::ContextSerializer < ActivityPub::Serializer
+  include RoutingHelper
+
+  attributes :id, :type, :attributed_to, :first
+
+  def type
+    'Collection'
+  end
+end
diff --git a/app/serializers/activitypub/note_serializer.rb b/app/serializers/activitypub/note_serializer.rb
index 8c44baf95f..20f23c0e8a 100644
--- a/app/serializers/activitypub/note_serializer.rb
+++ b/app/serializers/activitypub/note_serializer.rb
@@ -9,7 +9,7 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
              :in_reply_to, :published, :url,
              :attributed_to, :to, :cc, :sensitive,
              :atom_uri, :in_reply_to_atom_uri,
-             :conversation
+             :conversation, :context
 
   attribute :content
   attribute :content_map, if: :language?
@@ -163,6 +163,12 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
     end
   end
 
+  def context
+    return if object.conversation.nil?
+
+    ActivityPub::TagManager.instance.uri_for(object.conversation)
+  end
+
   def local?
     object.account.local?
   end
diff --git a/config/routes.rb b/config/routes.rb
index 49fcf3de79..26c5b3151f 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -120,6 +120,11 @@ Rails.application.routes.draw do
   end
 
   resource :inbox, only: [:create], module: :activitypub
+  resources :contexts, only: [:show], module: :activitypub do
+    member do
+      get :items
+    end
+  end
 
   constraints(encoded_path: /%40.*/) do
     get '/:encoded_path', to: redirect { |params|
diff --git a/db/migrate/20250828222741_add_parent_status_and_parent_account_to_conversations.rb b/db/migrate/20250828222741_add_parent_status_and_parent_account_to_conversations.rb
new file mode 100644
index 0000000000..e9e6a84b72
--- /dev/null
+++ b/db/migrate/20250828222741_add_parent_status_and_parent_account_to_conversations.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class AddParentStatusAndParentAccountToConversations < ActiveRecord::Migration[8.0]
+  def change
+    add_column :conversations, :parent_status_id, :bigint, null: true, default: nil
+    add_column :conversations, :parent_account_id, :bigint, null: true, default: nil
+  end
+end
diff --git a/db/migrate/20250902221600_add_index_on_conversation_to_statuses.rb b/db/migrate/20250902221600_add_index_on_conversation_to_statuses.rb
new file mode 100644
index 0000000000..9be338e8f6
--- /dev/null
+++ b/db/migrate/20250902221600_add_index_on_conversation_to_statuses.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class AddIndexOnConversationToStatuses < ActiveRecord::Migration[8.0]
+  disable_ddl_transaction!
+
+  def change
+    add_index :statuses, :conversation_id, algorithm: :concurrently
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 81b0fe7ccc..124eb0f358 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_08_20_084312) do
+ActiveRecord::Schema[8.0].define(version: 2025_09_02_221600) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -359,6 +359,8 @@ ActiveRecord::Schema[8.0].define(version: 2025_08_20_084312) do
     t.string "uri"
     t.datetime "created_at", precision: nil, null: false
     t.datetime "updated_at", precision: nil, null: false
+    t.bigint "parent_status_id"
+    t.bigint "parent_account_id"
     t.index ["uri"], name: "index_conversations_on_uri", unique: true, opclass: :text_pattern_ops, where: "(uri IS NOT NULL)"
   end
 
@@ -1145,6 +1147,7 @@ ActiveRecord::Schema[8.0].define(version: 2025_08_20_084312) do
     t.integer "quote_approval_policy", default: 0, null: false
     t.index ["account_id", "id", "visibility", "updated_at"], name: "index_statuses_20190820", order: { id: :desc }, where: "(deleted_at IS NULL)"
     t.index ["account_id"], name: "index_statuses_on_account_id"
+    t.index ["conversation_id"], name: "index_statuses_on_conversation_id"
     t.index ["deleted_at"], name: "index_statuses_on_deleted_at", where: "(deleted_at IS NOT NULL)"
     t.index ["id", "account_id"], name: "index_statuses_local_20190824", order: { id: :desc }, where: "((local OR (uri IS NULL)) AND (deleted_at IS NULL) AND (visibility = 0) AND (reblog_of_id IS NULL) AND ((NOT reply) OR (in_reply_to_account_id = account_id)))"
     t.index ["id", "language", "account_id"], name: "index_statuses_public_20250129", order: { id: :desc }, where: "((deleted_at IS NULL) AND (visibility = 0) AND (reblog_of_id IS NULL) AND ((NOT reply) OR (in_reply_to_account_id = account_id)))"
diff --git a/spec/fabricators/conversation_fabricator.rb b/spec/fabricators/conversation_fabricator.rb
index 5440e4380c..2d3f98c037 100644
--- a/spec/fabricators/conversation_fabricator.rb
+++ b/spec/fabricators/conversation_fabricator.rb
@@ -1,3 +1,5 @@
 # frozen_string_literal: true
 
-Fabricator(:conversation)
+Fabricator(:conversation) do
+  parent_account { Fabricate(:account) }
+end
diff --git a/spec/models/account_conversation_spec.rb b/spec/models/account_conversation_spec.rb
index 4e8727ca39..4ccbed0516 100644
--- a/spec/models/account_conversation_spec.rb
+++ b/spec/models/account_conversation_spec.rb
@@ -21,7 +21,7 @@ RSpec.describe AccountConversation do
 
     it 'appends to old record when there is a match' do
       last_status  = Fabricate(:status, account: alice, visibility: :direct)
-      conversation = described_class.create!(account: alice, conversation: last_status.conversation, participant_account_ids: [bob.id], status_ids: [last_status.id])
+      conversation = described_class.create!(account: alice, conversation_id: last_status.conversation_id, participant_account_ids: [bob.id], status_ids: [last_status.id])
 
       status = Fabricate(:status, account: bob, visibility: :direct, thread: last_status)
       status.mentions.create(account: alice)
@@ -36,7 +36,7 @@ RSpec.describe AccountConversation do
 
     it 'creates new record when new participants are added' do
       last_status  = Fabricate(:status, account: alice, visibility: :direct)
-      conversation = described_class.create!(account: alice, conversation: last_status.conversation, participant_account_ids: [bob.id], status_ids: [last_status.id])
+      conversation = described_class.create!(account: alice, conversation_id: last_status.conversation_id, participant_account_ids: [bob.id], status_ids: [last_status.id])
 
       status = Fabricate(:status, account: bob, visibility: :direct, thread: last_status)
       status.mentions.create(account: alice)
@@ -55,7 +55,7 @@ RSpec.describe AccountConversation do
     it 'updates last status to a previous value' do
       last_status  = Fabricate(:status, account: alice, visibility: :direct)
       status       = Fabricate(:status, account: alice, visibility: :direct)
-      conversation = described_class.create!(account: alice, conversation: last_status.conversation, participant_account_ids: [bob.id], status_ids: [status.id, last_status.id])
+      conversation = described_class.create!(account: alice, conversation_id: last_status.conversation_id, participant_account_ids: [bob.id], status_ids: [status.id, last_status.id])
       last_status.mentions.create(account: bob)
       last_status.destroy!
       conversation.reload
@@ -65,7 +65,7 @@ RSpec.describe AccountConversation do
 
     it 'removes the record if no other statuses are referenced' do
       last_status  = Fabricate(:status, account: alice, visibility: :direct)
-      conversation = described_class.create!(account: alice, conversation: last_status.conversation, participant_account_ids: [bob.id], status_ids: [last_status.id])
+      conversation = described_class.create!(account: alice, conversation_id: last_status.conversation_id, participant_account_ids: [bob.id], status_ids: [last_status.id])
       last_status.mentions.create(account: bob)
       last_status.destroy!
       expect(described_class.where(id: conversation.id).count).to eq 0
diff --git a/spec/requests/activitypub/contexts_spec.rb b/spec/requests/activitypub/contexts_spec.rb
new file mode 100644
index 0000000000..22ecea04eb
--- /dev/null
+++ b/spec/requests/activitypub/contexts_spec.rb
@@ -0,0 +1,127 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'ActivityPub Contexts' do
+  let(:conversation) { Fabricate(:conversation) }
+
+  describe 'GET #show' do
+    subject { get context_path(id: conversation.id), headers: nil }
+
+    let!(:status) { Fabricate(:status, conversation: conversation) }
+    let!(:unrelated_status) { Fabricate(:status) }
+
+    it 'returns http success and correct media type and correct items' do
+      subject
+
+      expect(response)
+        .to have_http_status(200)
+        .and have_cacheable_headers
+
+      expect(response.media_type)
+        .to eq 'application/activity+json'
+
+      expect(response.parsed_body[:type])
+        .to eq 'Collection'
+
+      expect(response.parsed_body[:first][:items])
+        .to be_an(Array)
+        .and have_attributes(size: 1)
+        .and include(ActivityPub::TagManager.instance.uri_for(status))
+        .and not_include(ActivityPub::TagManager.instance.uri_for(unrelated_status))
+    end
+
+    context 'with pagination' do
+      context 'with few statuses' do
+        before do
+          3.times do
+            Fabricate(:status, conversation: conversation)
+          end
+        end
+
+        it 'does not include a next page link' do
+          subject
+
+          expect(response.parsed_body[:first][:next]).to be_nil
+        end
+      end
+
+      context 'with many statuses' do
+        before do
+          (ActivityPub::ContextsController::DESCENDANTS_LIMIT + 1).times do
+            Fabricate(:status, conversation: conversation)
+          end
+        end
+
+        it 'includes a next page link' do
+          subject
+
+          expect(response.parsed_body['first']['next']).to_not be_nil
+        end
+      end
+    end
+  end
+
+  describe 'GET #items' do
+    subject { get items_context_path(id: conversation.id, page: 0, min_id: nil), headers: nil }
+
+    context 'with few statuses' do
+      before do
+        3.times do
+          Fabricate(:status, conversation: conversation)
+        end
+      end
+
+      it 'returns http success and correct media type and correct items' do
+        subject
+
+        expect(response)
+          .to have_http_status(200)
+
+        expect(response.media_type)
+          .to eq 'application/activity+json'
+
+        expect(response.parsed_body[:type])
+          .to eq 'Collection'
+
+        expect(response.parsed_body[:first][:items])
+          .to be_an(Array)
+          .and have_attributes(size: 3)
+
+        expect(response.parsed_body[:first][:next]).to be_nil
+      end
+    end
+
+    context 'with many statuses' do
+      before do
+        (ActivityPub::ContextsController::DESCENDANTS_LIMIT + 1).times do
+          Fabricate(:status, conversation: conversation)
+        end
+      end
+
+      it 'includes a next page link' do
+        subject
+
+        expect(response.parsed_body['first']['next']).to_not be_nil
+      end
+    end
+
+    context 'with page requested' do
+      before do
+        (ActivityPub::ContextsController::DESCENDANTS_LIMIT + 1).times do |_i|
+          Fabricate(:status, conversation: conversation)
+        end
+      end
+
+      it 'returns the correct items' do
+        get items_context_path(id: conversation.id, page: 0, min_id: nil), headers: nil
+        next_page = response.parsed_body['first']['next']
+        get next_page, headers: nil
+
+        expect(response.parsed_body['items'])
+          .to be_an(Array)
+          .and have_attributes(size: 1)
+      end
+    end
+  end
+end
diff --git a/spec/serializers/activitypub/note_serializer_spec.rb b/spec/serializers/activitypub/note_serializer_spec.rb
index d5d02a0d49..336f394337 100644
--- a/spec/serializers/activitypub/note_serializer_spec.rb
+++ b/spec/serializers/activitypub/note_serializer_spec.rb
@@ -23,6 +23,7 @@ RSpec.describe ActivityPub::NoteSerializer do
         'zh-TW' => a_kind_of(String),
       }),
       'replies' => replies_collection_values,
+      'context' => ActivityPub::TagManager.instance.uri_for(parent.conversation),
     })
   end