add fetch_related_entity callback

lib/diaspora_federation.rb

@@ -29,6 +29,7 @@ module DiasporaFederation
     fetch_author_public_key_by_entity_guid
     entity_author_is_local?
     fetch_entity_author_id_by_guid
+    fetch_related_entity
     queue_public_receive
     queue_private_receive
     receive_entity
@@ -181,6 +182,12 @@ module DiasporaFederation
     #   @param [String] guid of the entity
     #   @return [String] Diaspora ID of the person
     #
+    # fetch_related_entity
+    #   Fetches a related entity by a given guid
+    #   @param [String] entity_type (Post, Comment, Like, etc)
+    #   @param [String] guid of the entity
+    #   @return [DiasporaFederation::Entities::RelatedEntity] related entity
+    #
     # queue_public_receive
     #   Queue a public salmon xml to process in background
     #   @param [String] data salmon slap xml or magic envelope xml

test/dummy/config/initializers/diaspora_federation.rb

@@ -90,6 +90,16 @@ DiasporaFederation.configure do |config|
       Entity.where(entity_type: entity_type, guid: guid).joins(:author).pluck(:diaspora_id).first
     end
 
+    on :fetch_related_entity do |entity_type, guid|
+      entity = Entity.find_by(entity_type: entity_type, guid: guid)
+      if entity
+        DiasporaFederation::Entities::RelatedEntity.new(
+          author: entity.author.diaspora_id,
+          local:  !entity.author.serialized_private_key.nil?
+        )
+      end
+    end
+
     on :queue_public_receive do
     end