Remove unused ActivityPub @context values depending on response (#10378)

Fix #8078

app/lib/activitypub/adapter.rb

@@ -1,30 +1,23 @@
 # frozen_string_literal: true
 
 class ActivityPub::Adapter < ActiveModelSerializers::Adapter::Base
-  CONTEXT = {
-    '@context': [
-      'https://www.w3.org/ns/activitystreams',
-      'https://w3id.org/security/v1',
+  NAMED_CONTEXT_MAP = {
+    activitystreams: 'https://www.w3.org/ns/activitystreams',
+    security: 'https://w3id.org/security/v1',
+  }.freeze
 
-      {
-        'manuallyApprovesFollowers' => 'as:manuallyApprovesFollowers',
-        'sensitive'                 => 'as:sensitive',
-        'movedTo'                   => { '@id' => 'as:movedTo', '@type' => '@id' },
-        'alsoKnownAs'               => { '@id' => 'as:alsoKnownAs', '@type' => '@id' },
-        'Hashtag'                   => 'as:Hashtag',
-        'ostatus'                   => 'http://ostatus.org#',
-        'atomUri'                   => 'ostatus:atomUri',
-        'inReplyToAtomUri'          => 'ostatus:inReplyToAtomUri',
-        'conversation'              => 'ostatus:conversation',
-        'toot'                      => 'http://joinmastodon.org/ns#',
-        'Emoji'                     => 'toot:Emoji',
-        'focalPoint'                => { '@container' => '@list', '@id' => 'toot:focalPoint' },
-        'featured'                  => { '@id' => 'toot:featured', '@type' => '@id' },
-        'schema'                    => 'http://schema.org#',
-        'PropertyValue'             => 'schema:PropertyValue',
-        'value'                     => 'schema:value',
-      },
-    ],
+  CONTEXT_EXTENSION_MAP = {
+    manually_approves_followers: { 'manuallyApprovesFollowers' => 'as:manuallyApprovesFollowers' },
+    sensitive: { 'sensitive' => 'as:sensitive' },
+    hashtag: { 'Hashtag' => 'as:Hashtag' },
+    moved_to: { 'movedTo' => { '@id' => 'as:movedTo', '@type' => '@id' } },
+    also_known_as: { 'alsoKnownAs' => { '@id' => 'as:alsoKnownAs', '@type' => '@id' } },
+    emoji: { 'toot' => 'http://joinmastodon.org/ns#', 'Emoji' => 'toot:Emoji' },
+    featured: { 'toot' => 'http://joinmastodon.org/ns#', 'featured' => { '@id' => 'toot:featured', '@type' => '@id' } },
+    property_value: { 'schema' => 'http://schema.org#', 'PropertyValue' => 'schema:PropertyValue', 'value' => 'schema:value' },
+    atom_uri: { 'ostatus' => 'http://ostatus.org#', 'atomUri' => 'ostatus:atomUri' },
+    conversation: { 'ostatus' => 'http://ostatus.org#', 'inReplyToAtomUri' => 'ostatus:inReplyToAtomUri', 'conversation' => 'ostatus:conversation' },
+    focal_point: { 'toot' => 'http://joinmastodon.org/ns#', 'focalPoint' => { '@container' => '@list', '@id' => 'toot:focalPoint' } },
   }.freeze
 
   def self.default_key_transform
@@ -36,8 +29,36 @@ class ActivityPub::Adapter < ActiveModelSerializers::Adapter::Base
   end
 
   def serializable_hash(options = nil)
-    options = serialization_options(options)
-    serialized_hash = ActiveModelSerializers::Adapter::Attributes.new(serializer, instance_options).serializable_hash(options)
-    CONTEXT.merge(self.class.transform_key_casing!(serialized_hash, instance_options))
+    options         = serialization_options(options)
+    serialized_hash = serializer.serializable_hash(options)
+    serialized_hash = self.class.transform_key_casing!(serialized_hash, instance_options)
+
+    { '@context' => serialized_context }.merge(serialized_hash)
+  end
+
+  private
+
+  def serialized_context
+    context_array = []
+
+    serializer_options = serializer.send(:instance_options) || {}
+    named_contexts     = [:activitystreams] + serializer._named_contexts.keys + serializer_options.fetch(:named_contexts, {}).keys
+    context_extensions = serializer._context_extensions.keys + serializer_options.fetch(:context_extensions, {}).keys
+
+    named_contexts.each do |key|
+      context_array << NAMED_CONTEXT_MAP[key]
+    end
+
+    extensions = context_extensions.each_with_object({}) do |key, h|
+      h.merge!(CONTEXT_EXTENSION_MAP[key])
+    end
+
+    context_array << extensions unless extensions.empty?
+
+    if context_array.size == 1
+      context_array.first
+    else
+      context_array
+    end
   end
 end

app/lib/activitypub/serializer.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class ActivityPub::Serializer < ActiveModel::Serializer
+  with_options instance_writer: false, instance_reader: true do |serializer|
+    serializer.class_attribute :_named_contexts
+    serializer.class_attribute :_context_extensions
+
+    self._named_contexts     ||= {}
+    self._context_extensions ||= {}
+  end
+
+  def self.inherited(base)
+    super
+
+    base._named_contexts     = _named_contexts.dup
+    base._context_extensions = _context_extensions.dup
+  end
+
+  def self.context(*named_contexts)
+    named_contexts.each do |context|
+      _named_contexts[context] = true
+    end
+  end
+
+  def self.context_extensions(*extension_names)
+    extension_names.each do |extension_name|
+      _context_extensions[extension_name] = true
+    end
+  end
+end