README.md

To see it fail:

BUNDLE_GEMFILE=Gemfile-214 bundle install
BUNDLE_GEMFILE=Gemfile-299 bundle install

BUNDLE_GEMFILE=Gemfile-214 bundle exec rspec serialization_spec.rb
=> passes
BUNDLE_GEMFILE=Gemfile-299 bundle exec rspec serialization_spec.rb
=> fails

Gemfile-214

source 'https://rubygems.org'

gem 'sqlite3'
gem 'rspec', '2.14'
gem 'activerecord'

Gemfile-214.lock

GEM
  remote: https://rubygems.org/
  specs:
    activemodel (4.1.6)
      activesupport (= 4.1.6)
      builder (~> 3.1)
    activerecord (4.1.6)
      activemodel (= 4.1.6)
      activesupport (= 4.1.6)
      arel (~> 5.0.0)
    activesupport (4.1.6)
      i18n (~> 0.6, >= 0.6.9)
      json (~> 1.7, >= 1.7.7)
      minitest (~> 5.1)
      thread_safe (~> 0.1)
      tzinfo (~> 1.1)
    arel (5.0.1.20140414130214)
    builder (3.2.2)
    diff-lcs (1.2.5)
    i18n (0.6.11)
    json (1.8.1)
    minitest (5.4.1)
    rspec (2.14.0)
      rspec-core (~> 2.14.0)
      rspec-expectations (~> 2.14.0)
      rspec-mocks (~> 2.14.0)
    rspec-core (2.14.8)
    rspec-expectations (2.14.5)
      diff-lcs (>= 1.1.3, < 2.0)
    rspec-mocks (2.14.6)
    sqlite3 (1.3.9)
    thread_safe (0.3.4)
    tzinfo (1.2.2)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord
  rspec (= 2.14)
  sqlite3

Gemfile-299

source 'https://rubygems.org'

gem 'sqlite3'
gem 'rspec', '2.99'
gem 'activerecord'

Gemfile-299.lock

GEM
  remote: https://rubygems.org/
  specs:
    activemodel (4.1.6)
      activesupport (= 4.1.6)
      builder (~> 3.1)
    activerecord (4.1.6)
      activemodel (= 4.1.6)
      activesupport (= 4.1.6)
      arel (~> 5.0.0)
    activesupport (4.1.6)
      i18n (~> 0.6, >= 0.6.9)
      json (~> 1.7, >= 1.7.7)
      minitest (~> 5.1)
      thread_safe (~> 0.1)
      tzinfo (~> 1.1)
    arel (5.0.1.20140414130214)
    builder (3.2.2)
    diff-lcs (1.2.5)
    i18n (0.6.11)
    json (1.8.1)
    minitest (5.4.1)
    rspec (2.99.0)
      rspec-core (~> 2.99.0)
      rspec-expectations (~> 2.99.0)
      rspec-mocks (~> 2.99.0)
    rspec-core (2.99.2)
    rspec-expectations (2.99.2)
      diff-lcs (>= 1.1.3, < 2.0)
    rspec-mocks (2.99.2)
    sqlite3 (1.3.9)
    thread_safe (0.3.4)
    tzinfo (1.2.2)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord
  rspec (= 2.99)
  sqlite3

serialization_spec.rb

require 'active_record'
require 'sqlite3'

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

ActiveRecord::Schema.define do
  create_table :things, force: true do |t|
    t.text :something_hash
  end
end

class Thing < ActiveRecord::Base
  serialize :something_hash
end

describe 'serializing doubles' do
  it 'works' do
    something = double("something")
    Thing.create! something_hash: something
  end
end