gistfile1.rb

# Activate the gem you are reporting the issue against.
unless File.exists?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.0.0'
    gem 'pg'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'

require 'minitest/autorun'
require 'logger'

USERNAME = ENV["USERNAME"]
PASSWORD = ENV["PASSWORD"]
DB       = ENV["DB"]

ActiveRecord::Base.establish_connection("postgres://#{USERNAME}:#{PASSWORD}@localhost/#{DB}")
ActiveRecord::Base.logger = Logger.new(STDOUT)

unless ActiveRecord::Base.connection.table_exists? "products"
  ActiveRecord::Schema.define do
    create_table :products do |t|
      t.json :data
    end
  end
end

class Product < ActiveRecord::Base
end

class BugTest < MiniTest::Unit::TestCase
  def test_updating_json_column
    Product.create!(:data => { :size => "Small" })

    product = Product.last
    product.data[:size] = "Large"

    assert product.changed?, 'failed to detect json column change'
  end
end

When update or set something, a workaround is to manually call: attr_name_will_change!.