Fix #4607 - Accept/reject activities use FollowRequest, which has inverse relations (#4616)

app/serializers/activitypub/accept_follow_serializer.rb

@@ -3,7 +3,7 @@
 class ActivityPub::AcceptFollowSerializer < ActiveModel::Serializer
   attributes :id, :type, :actor
 
-  has_one :object, serializer: ActivityPub::FollowSerializer
+  has_one :object, serializer: ActivityPub::InverseFollowSerializer
 
   def id
     [ActivityPub::TagManager.instance.uri_for(object.target_account), '#accepts/follows/', object.id].join

app/serializers/activitypub/inverse_follow_serializer.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class ActivityPub::InverseFollowSerializer < ActiveModel::Serializer
+  attributes :id, :type, :actor
+  attribute :virtual_object, key: :object
+
+  def id
+    [ActivityPub::TagManager.instance.uri_for(object.target_account), '#follows/', object.id].join
+  end
+
+  def type
+    'Follow'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.target_account)
+  end
+
+  def virtual_object
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+end

app/serializers/activitypub/reject_follow_serializer.rb

@@ -3,7 +3,7 @@
 class ActivityPub::RejectFollowSerializer < ActiveModel::Serializer
   attributes :id, :type, :actor
 
-  has_one :object, serializer: ActivityPub::FollowSerializer
+  has_one :object, serializer: ActivityPub::InverseFollowSerializer
 
   def id
     [ActivityPub::TagManager.instance.uri_for(object.target_account), '#rejects/follows/', object.id].join