small_rails.rb

#!/usr/bin/env ruby

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  gem 'sqlite3'
  gem 'activerecord', '4.2.4'
  gem 'rspec', '3.3.0'
  gem 'shoulda-matchers', '3.0.0'
end

require 'active_record'
require 'rspec/autorun'
require 'logger'

Shoulda::Matchers.configure do |config|
  config.integrate do |with|
    with.test_framework :rspec
    with.library :active_record
    with.library :active_model
  end
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.integer :comment_count
  end
end

class Post < ActiveRecord::Base
  validates_numericality_of :comment_count, only_integer: true, greater_than_or_equal_to: 0, allow_nil: true
end

RSpec.describe Post, type: :model do
  it { should validate_numericality_of(:comment_count).only_integer.is_greater_than_or_equal_to(0).allow_nil }
end

Copied from: thoughtbot/shoulda-matchers#784 (comment)