Reproducing without additional gems and with sqlite

gistfile1.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.0.0'
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
  exec_query "PRAGMA foreign_keys = ON;"

  create_table :posts do |t|
  end

  exec_query  <<-eos
    create table comments(
      id int,
      post_id,
      PRIMARY KEY (id),
      FOREIGN KEY (post_Id) REFERENCES posts(id)
    );
  eos
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