Regression: Default value for composed-of aggregate field ignored in ActiveRecord 6.1.0

[mintyfresh]

Steps to reproduce

• Create an aggregate field on a model using composed_of
• Set a default value for the field using attribute

This was encountered when upgrading an existing application from 6.0.3.4 to 6.1.0.

Executable Test Case

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "6.1.0"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :currency
  end
end

class Currency
  attr_reader :id

  def initialize(currency)
    @id = currency
  end

  def ==(other)
    other.is_a?(Currency) && other.id == id
  end
end

class User < ActiveRecord::Base
  attribute :currency, :string, default: -> { 'CAD' }

  composed_of :currency, allow_nil:  true,
                         class_name: 'Currency',
                         converter:  -> (value) { Currency.new(value) },
                         mapping:    [:currency, :id]
end

class BugTest < Minitest::Test
  def test_default_value
    assert_equal Currency.new('CAD'), User.new.currency
  end
end

Expected behavior

Attribute should have default value wrapped in aggregate class:

# Running:

.

Finished in 0.003190s, 313.5189 runs/s, 313.5189 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Actual behavior

Attribute has no default value:

# Running:

F

Finished in 0.003208s, 311.7013 runs/s, 311.7013 assertions/s.

  1) Failure:
BugTest#test_default_value [testcase.rb:52]:
Expected: #<Currency:0x00005598e7fd9320 @id="CAD">
  Actual: nil

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

NOTE: Changing the activerecord gem version to 6.0.3.4 will make this test case pass, so this appears to be a regression.

System configuration

Rails version: 6.1.0

Ruby version: 2.6.6