Add Conversation#to_message_id for class/id/date portion of conversation thread header

app/mailers/notification_mailer.rb

@@ -84,9 +84,9 @@ class NotificationMailer < ApplicationMailer
   def thread_by_conversation(conversation)
     return if conversation.nil?
 
-    msg_id = "<conversation-#{conversation.id}.#{conversation.created_at.strftime('%Y-%m-%d')}@#{Rails.configuration.x.local_domain}>"
+    message_id = "<#{conversation.to_message_id}@#{Rails.configuration.x.local_domain}>"
 
-    headers['In-Reply-To'] = msg_id
+    headers['In-Reply-To'] = message_id
-    headers['References']  = msg_id
+    headers['References'] = message_id
   end
 end

app/models/conversation.rb

@@ -18,4 +18,8 @@ class Conversation < ApplicationRecord
   def local?
     uri.nil?
   end
+
+  def to_message_id
+    "#{self.class.name.downcase}-#{id}.#{created_at.to_date}"
+  end
 end

spec/models/conversation_spec.rb

@@ -12,4 +12,13 @@ RSpec.describe Conversation do
       expect(Fabricate(:conversation, uri: 'abc').local?).to be false
     end
   end
+
+  describe '#to_message_id' do
+    it 'converts the conversation details into a string ID' do
+      conversation = described_class.new(id: 123, created_at: DateTime.new(2024, 1, 1))
+
+      expect(conversation.to_message_id)
+        .to eq('conversation-123.2024-01-01')
+    end
+  end
 end