rubyquicktips:

In Rails 2 the only way to add multiple validations to a field is through separate validate statements:

validates_presence_of :title
validates_length_of :title, :maximum => 30

Rails 3 simplifies this process by adding a method called validates which is a “shortcut to all default validators”. Using the validates method your code will look like this:

validates(:title, :presence => true, :length => {:maximum => 30})

You can find more information about this method in the API documentation here.

(Source: rubyquicktips)