Model validation in rails

person.rb

class Person <  ActiveRecord::Base
  attr_accessible :username, :password, :age, :graduation_year
  
  #must have first and last name. 
  #If not present, throw an error message (optional)
  validates_presence_of :username

  
  validates_presence_of :password


  validates_length_of :password

  validates_format_of :password, 
    :with => /[0-9]/
    
  # Use :scope if the combination of username/password must be unique.
  validates_uniqueness_of :username, 
    :case_sensitive => false,
    :scope => [:password]
  
  #age ain't nothing but a number  
  validates_numericality_of :age, 
    :allow_nil => true
  
  #so is graduation year
  validates_numericality_of :graduation_year, 
    :allow_nil => true,
    :greater_than => 2012, 
    :less_than_or_equal_to => Time.now.year,
    :only_integer => true
  
end