gistfile1.rb

# /app/models/product.rb
validates :title, :description, :image_url, :presence => true
validates :price, :numericality => {:greater_than_or_equal_to => 0.01}
validates :title, :uniqueness   => true, 
                  :length       => {:minimum => minimum = 10,
                                    :message => "is too short. Must be at least #{minimum} characters."}
validates :image_url, :format => {:with    => %r{\.(gif|jpg|png)$}i,
                                  :message => 'must be a URL for GIF, JPG, or PNG image.'
}

# /test/unit/product_test.rb
test "product title must be at least 10 characters long" do
  product = Product.new(:description => "yyy",
                        :price       => 1,
                        :image_url   => "fred.gif")
  product.title = "Incorrect"
  assert product.invalid?, '1 - Title length is wrong'

  product.title = "And this"
  assert product.invalid?, '2 - Title length is wrong'

  product.title = 'At least 10 characters long is correct'
  assert product.valid?
end