Migration of notifications to mailboxer, without sending new emails

app/models/notification.rb

@@ -4,6 +4,8 @@ class Notification < ApplicationRecord
   belongs_to :post, optional: true
 
   validates :subject, length: { maximum: 255 }
+  validates :subject, presence: true
+  validates :body, presence: true
 
   scope :unread, -> { where(read: false) }
   scope :by_recipient, ->(recipient) { where(recipient_id: recipient) }

db/migrate/20190720000625_notifications_to_mailboxer.rb

@@ -1,7 +1,27 @@
 class NotificationsToMailboxer < ActiveRecord::Migration[5.2]
-  def change
+  def up
     Notification.all.each do |n|
-      n.sender.send_message(n.recipient, n.body, n.subject)
+      next unless n.valid?
+
+      conversation = Mailboxer::ConversationBuilder.new(
+        subject:    n.subject,
+        created_at: n.created_at,
+        updated_at: n.updated_at
+      ).build
+
+      message = Mailboxer::MessageBuilder.new(
+        sender:       n.sender,
+        conversation: conversation,
+        recipients:   [n.recipient],
+        body:         n.body,
+        subject:      n.subject,
+        # attachment:   attachment,
+        created_at:   n.created_at,
+        updated_at:   n.updated_at
+      ).build
+
+      conversation.save!
+      message.save!
     end
   end
 end