gistfile1.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.2.0'
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 do |t|
    t.text :bar
  end
end

class Post < ActiveRecord::Base
  serialize :bar, Hash
  attr_accessor :bar

  after_initialize -> {
    self.bar.merge!(foo: "bar")
  }
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.new

    expected = {foo: "bar"}
    assert_equal expected, post.bar
  end
end


$ ruby rails-4.2-test.rb 
-- create_table(:posts)
D, [2015-02-14T11:44:49.413052 #60774] DEBUG -- :    (0.4ms)  CREATE TABLE "posts" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "bar" text) 
   -> 0.0024s
Run options: --seed 61615

# Running:

E

Finished in 0.009806s, 101.9784 runs/s, 0.0000 assertions/s.

  1) Error:
BugTest#test_association_stuff:
NoMethodError: undefined method `merge!' for nil:NilClass
    ../rails-4.2-test.rb:25:in `block in <class:Post>'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:441:in `instance_exec'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:441:in `block in make_lambda'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:225:in `call'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:225:in `block in halting_and_conditional'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:92:in `call'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:92:in `_run_callbacks'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activesupport-4.2.0/lib/active_support/callbacks.rb:734:in `_run_initialize_callbacks'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activerecord-4.2.0/lib/active_record/core.rb:283:in `initialize'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activerecord-4.2.0/lib/active_record/inheritance.rb:61:in `new'
    /Users/jarmo/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/activerecord-4.2.0/lib/active_record/inheritance.rb:61:in `new'
    ../rails-4.2-test.rb:31:in `test_association_stuff'

1 runs, 0 assertions, 0 failures, 1 errors, 0 skips