rails--rails/activemodel/lib/active_model/validations/presence.rb

require 'active_support/core_ext/object/blank'

module ActiveModel
  # == Active Model Presence Validator
  module Validations
    class PresenceValidator < EachValidator
      def validate(record)
        record.errors.add_on_blank(attributes, options)
      end
    end

    module HelperMethods
      # Validates that the specified attributes are not blank (as defined by
      # Object#blank?). Happens by default on save.
      #
      #   class Person < ActiveRecord::Base
      #     validates_presence_of :first_name
      #   end
      #
      # The first_name attribute must be in the object and it cannot be blank.
      #
      # If you want to validate the presence of a boolean field (where the real values
      # are true and false), you will want to use
      # <tt>validates_inclusion_of :field_name, :in => [true, false]</tt>.
      #
      # This is due to the way Object#blank? handles boolean values:
      # <tt>false.blank? # => true</tt>.
      #
      # Configuration options:
      # * <tt>:message</tt> - A custom error message (default is: "can't be blank").
      # * <tt>:on</tt> - Specifies when this validation is active. Runs in all
      #   validation contexts by default (+nil+), other options are <tt>:create</tt>
      #   and <tt>:update</tt>.
      # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if
      #   the validation should occur (e.g. <tt>:if => :allow_validation</tt>, or
      #   <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>). The method, proc
      #   or string should return or evaluate to a true or false value.
      # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine
      #   if the validation should not occur (e.g. <tt>:unless => :skip_validation</tt>,
      #   or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>). The method,
      #   proc or string should return or evaluate to a true or false value.
      # * <tt>:strict</tt> - Specifies whether validation should be strict. 
      #   See <tt>ActiveModel::Validation#validates!</tt> for more information.
      def validates_presence_of(*attr_names)
        validates_with PresenceValidator, _merge_attributes(attr_names)
      end
    end
  end
end