Foreign key violation, caused by incorrect order of destroying linked objects (link is 'belongs_to')

gistfile1.rb

gem 'activerecord', '4.0.0'
require 'active_record'
require 'minitest/autorun'
require 'logger'

gem "schema_plus"
require 'schema_plus'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(
  adapter: 'postgresql',
  host: 'localhost',
  port: '5432',
  username: 'postgres',
  password: '123123',
  database: 'dev',
  schema_search_path: 'public',
  encoding: 'utf8',
  template: 'template1'
)
ActiveRecord::Base.logger = Logger.new(STDOUT)

class ActiveRecord::Schema
  include ::SchemaPlus::ActiveRecord::ConnectionAdapters
end

ActiveRecord::Schema.define do
  drop_table :comments, if_exists: true
  drop_table :posts , if_exists: true

  create_table :posts do |t|
  end

  create_table :comments do |t|
    t.integer :post_id
  end

  add_foreign_key :comments, :post_id, :posts, :id
end

class Post < ActiveRecord::Base
end

class Comment < ActiveRecord::Base
  belongs_to :post, :dependent => :destroy
end

class BugTest < MiniTest::Unit::TestCase
  def test_association_stuff
    post = Post.create!

    comment = Comment.new
    comment.post = post
    comment.save!

    comment.destroy!
  end
end