Fix owned account notes not being deleted when an account is deleted (#16579)

* Add account_notes relationship

* Add tests

* Fix owned account notes not being deleted when an account is deleted

* Add post-migration to clean up orphaned account notes

db/post_migrate/20210808071221_clear_orphaned_account_notes.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class ClearOrphanedAccountNotes < ActiveRecord::Migration[5.2]
+  class Account < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+  end
+
+  class AccountNote < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account
+    belongs_to :target_account, class_name: 'Account'
+  end
+
+  def up
+    AccountNote.where('NOT EXISTS (SELECT * FROM users u WHERE u.account_id = account_notes.account_id)').in_batches.delete_all
+  end
+
+  def down
+    # nothing to do
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_06_30_000137) do
+ActiveRecord::Schema.define(version: 2021_08_08_071221) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"