Remapping type column values using the Rails 5 Attributes API, good for both STI and polymorphic belongs_to

attachable_sti_type.rb

# lib/attachable_sti_type.rb

class AttachableStiType < ActiveRecord::Type::String
  def cast_value(value)
    if value =~ /\A([a-z]+)\/([a-z]+)\Z/
      "#{$2}/#{$1}_record".classify
    else
      super
    end
  end

  def serialize(value)
    if value && value.underscore =~ /\A([a-z]+)\/([a-z]+)_record\Z/
      "#{$2}/#{$1}"
    else
      super
    end
  end

  def changed_in_place?(original_value_for_database, value)
    original_value_for_database != serialize(value)
  end
end

attachment.rb

# app/models/attachment.rb

class Attachment < ApplicationRecord
  belongs_to :document, polymorphic: true
  belongs_to :attachable, polymorphic: true, dependent: :destroy, autosave: true
  attribute :attachable_type, :attachable_sti_type
  #...
end

types.rb

# config/initializers/types.rb

require 'attachable_sti_type'
ActiveRecord::Type.register(:attachable_sti_type, AttachableStiType)