Gemfile

source 'https://rubygems.org'

gem 'pg'
gem 'activerecord'

write_without_typecast.rb

require 'rubygems'
require 'bundler/setup'

require 'pg'
require 'active_record'

require 'minitest/autorun'

ActiveRecord::Base.establish_connection(
  adapter:  'postgresql',
  host:     'localhost',
  username: 'postgres',
  password: '',
  database: 'testy_test'
)


describe 'json typecasting' do

  class Post < ActiveRecord::Base

    validate :data_is_valid_json

    def data=(str)
      begin
        super(str)
      rescue JSON::ParserError
        @__original_data_json_string = str
      end
    end

    private

    def data_is_valid_json
      return unless @__original_data_json_string
      
      begin
        JSON.parse(@__original_data_json_string)
      rescue JSON::ParserError
        errors.add :data, 'invalid JSON'
      end
    end

  end

  before do
    ActiveRecord::Base.connection.create_table :posts do |t|
      t.json :data
    end
  end

  after do
    ActiveRecord::Base.connection.drop_table :posts
  end

  it 'persists using real json' do
    p = Post.create data: "{\"foo\" : \"bar\"}"

    assert p.persisted?
    assert_equal({'foo' => 'bar'}, p.data)
  end

  it 'adds an error when using invalid json' do
    p = Post.create data: "invalid json"

    assert p.errors.any?
  end
end