Assigning PROs to polymorphic associations

app.rb

require 'active_record'

ActiveRecord::Base.logger = Logger.new(STDERR)

ActiveRecord::Base.establish_connection(
  adapter: "sqlite3",
  database: ":memory:"
)

ActiveRecord::Schema.define do
  create_table :audits do |table|
    table.column :user_id, :integer
    table.column :user_type, :string
  end
end

class Audit < ActiveRecord::Base
  belongs_to :user, polymorphic: true
end

class User
  include ActiveModel::Conversion
  include ActiveModel::Validations
  extend  ActiveModel::Naming

  def initialize
    @attributes = AttributeSet.new({ 'id' => 42 })
  end

  def attributes
    @attributes
  end

  def self.primary_key
    'id'
  end

  def id
    attributes['id']
  end

  def persisted?
    false
  end
end

audit = Audit.new
audit.user = User.new
audit.save!

Gemfile

# A sample Gemfile
source "https://rubygems.org"

gem "activerecord"
gem "sqlite3"