ActiveRecord 4 save generates incorrect INSERT statement after deserialization

active_record_gem.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  # Activate the gem you are reporting the issue against.
  gem "activerecord", "4.2.7.1"
  # gem "activerecord", "5.0.0" # Fixed in 5.0 :)
  gem "sqlite3"
end

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

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# 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 :posts, force: true do |t|
    t.text :mapped_data
  end

  create_table :comments, force: true do |t|
    t.string :title, null: false
    t.timestamps null: false
  end
end

class Post < ActiveRecord::Base
  serialize :mapped_data, Hash
end

class Comment < ActiveRecord::Base
  validates :title, presence: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    hash = {
      foo: "bar",
      animal: 3,
      comment: Comment.new(title: "yaay")
    }

    post = Post.create!(mapped_data: hash)
    assert_equal 1, Post.count

    comment = post.reload.mapped_data[:comment]
    assert_equal "yaay", comment.title
    assert comment.valid?
    comment.save! # fails with SQLite3::ConstraintException
    assert_equal 1, Comment.count
  end
end