title: undefined method `assert' in ruby on rails
body: <p>Getting this error message after running the command </p>
  
  <p>rake test:units</p>
  
  <pre><code> undefined method `assert' for ProductTest:Class (NoMethodError)
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:251:in `require'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:251:in `require'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:236:in `load_dependency'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:251:in `require'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb:10
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb:9:in `each'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb:9
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb:4:in `select'
      from /home/mayank/src/proj/vendor/bundle/ruby/1.8/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb:4
  rake aborted!
  </code></pre>
  
  <p>Source code of the application is as below.  Its giving error on line assert. </p>
  
  <pre><code>require 'test_helper'
  
  class ProductTest &lt; ActiveSupport::TestCase
    product = Product.new
    assert product.invalid?
    assert product.errors[:title].any?
    assert product.errors[:description].any?
    assert product.errors[:price].any?
    assert product.errors[:image_url].any?
    # test "the truth" do
    #   assert true
    # end
  end
  </code></pre>