How to Wrap ActiveRecord attr_encrypted Setter without Causing Infinite Loop in Ruby on Rails

I’m working with a Rails application that uses the attr_encrypted gem to handle encryption for sensitive data. Previously, we only stored the last 4 digits of the SSN. We’re migrating to store the full SSN while keeping the last 4 digits accessible as ssn_last_four for legacy compatibility.

class Person < ApplicationRecord
  prepend SsnLastFourConcern

  attr_encrypted :ssn, key: 'some_secret_key'
  attr_encrypted :ssn, key: 'some_secret_key'
end

and I have the concern:

module SsnLastFourConcern
  extend ActiveSupport::Concern

  def ssn_last_four
    return ssn[-4..] if ssn?
    super
  end

  def ssn=(value)
    self.ssn_last_four = value[-4..] if value.present? && value.length == 9
    super(value)
  end
end

The issue is that calling super(value) in the overridden ssn= setter leads to an infinite loop. But calling the super in ssn_last_four it works as expected

The workaround could be to use the before_save callback on ssn_last_four and make sure it reflects SSN. Also, by overriding the getter, the rest of the app will work properly, but I want to make sure that in case someone gets encrypted data from the column and manually decrypts it, he still gets the valid data.

EDIT: I do not override the ssn_last_four setter.

EDIT: Removed extend ActiveSupport::Concern as @engineersmnky suggested

Here is the stack trace: SystemStackError: stack level too deep from ruby-2.7.8/gems/activemodel-6.0.4/lib/active_model/attribute_set.rb:33:in `key?'