Add `ActiveModel::Validations::AbsenceValidator`, a validator to check the absence of attributes.

Add `ActiveModel::Errors#add_on_present` method. Adds error messages to present attributes.

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
+*   Add `ActiveModel::Validations::AbsenceValidator`, a validator to check the
+    absence of attributes.
+
+        class Person < ActiveRecord::Base
+          validates_absence_of :first_name
+        end
+
+        person = Person.new
+        person.first_name = "John"
+        person.valid?
+        => false
+        # first_name must be blank
+
+    * Roberto Vasquez Angel*
+
+*   Added `ActiveModel::Errors#add_on_present` method. Adds error messages to
+    present attributes.
+
+    *Roberto Vasquez Angel*
 
 *   `[attribute]_changed?` now returns `false` after a call to `reset_[attribute]!`
 

activemodel/lib/active_model/errors.rb

@@ -328,6 +328,19 @@ def add_on_blank(attributes, options = {})
       end
     end
 
+    # Will add an error message to each of the attributes in +attributes+ that
+    # is present (using Object#present?).
+    #
+    #   person.errors.add_on_present(:name)
+    #   person.errors.messages
+    #   # => { :name => ["must be blank"] }
+    def add_on_present(attributes, options = {})
+      Array(attributes).flatten.each do |attribute|
+        value = @base.send(:read_attribute_for_validation, attribute)
+        add(attribute, :not_blank, options) if value.present?
+      end
+    end
+
     # Returns +true+ if an error on the attribute with the given message is
     # present, +false+ otherwise. +message+ is treated the same as for +add+.
     #

activemodel/lib/active_model/locale/en.yml

@@ -13,6 +13,7 @@ en:
       accepted: "must be accepted"
       empty: "can't be empty"
       blank: "can't be blank"
+      not_blank: "must be blank"      
       too_long: "is too long (maximum is %{count} characters)"
       too_short: "is too short (minimum is %{count} characters)"
       wrong_length: "is the wrong length (should be %{count} characters)"

activemodel/lib/active_model/validations/absence.rb

+module ActiveModel
+  module Validations
+    # == Active Model Absence Validator
+    class AbsenceValidator < EachValidator #:nodoc:
+      def validate(record)
+        record.errors.add_on_present(attributes, options)
+      end
+    end
+
+    module HelperMethods
+      # Validates that the specified attributes are blank (as defined by
+      # Object#blank?). Happens by default on save.
+      #
+      #   class Person < ActiveRecord::Base
+      #     validates_absence_of :first_name
+      #   end
+      #
+      # The first_name attribute must be in the object and it must be blank.
+      #
+      # Configuration options:
+      # * <tt>:message</tt> - A custom error message (default is: "must be blank").
+      #
+      # There is also a list of default options supported by every validator:
+      # +:if+, +:unless+, +:on+ and +:strict+.
+      # See <tt>ActiveModel::Validation#validates</tt> for more information
+      def validates_absence_of(*attr_names)
+        validates_with AbsenceValidator, _merge_attributes(attr_names)
+      end
+    end
+  end
+end

activemodel/test/cases/validations/absence_validation_test.rb

+# encoding: utf-8
+require 'cases/helper'
+require 'models/topic'
+require 'models/person'
+require 'models/custom_reader'
+
+class AbsenceValidationTest < ActiveModel::TestCase
+  teardown do
+    Topic.reset_callbacks(:validate)
+    Person.reset_callbacks(:validate)
+    CustomReader.reset_callbacks(:validate)
+  end
+
+  def test_validate_absences
+    Topic.validates_absence_of(:title, :content)
+    t = Topic.new
+    t.title = "foo"
+    t.content = "bar"
+    assert t.invalid?
+    assert_equal ["must be blank"], t.errors[:title]
+    assert_equal ["must be blank"], t.errors[:content]
+    t.title = ""
+    t.content  = "something"
+    assert t.invalid?
+    assert_equal ["must be blank"], t.errors[:content]
+    t.content = ""
+    assert t.valid?
+  end
+
+  def test_accepts_array_arguments
+    Topic.validates_absence_of %w(title content)
+    t = Topic.new
+    t.title = "foo"
+    t.content = "bar"
+    assert t.invalid?
+    assert_equal ["must be blank"], t.errors[:title]
+    assert_equal ["must be blank"], t.errors[:content]
+  end
+
+  def test_validates_acceptance_of_with_custom_error_using_quotes
+    Person.validates_absence_of :karma, message: "This string contains 'single' and \"double\" quotes"
+    p = Person.new
+    p.karma = "good"
+    assert p.invalid?
+    assert_equal "This string contains 'single' and \"double\" quotes", p.errors[:karma].last
+  end
+
+  def test_validates_absence_of_for_ruby_class
+    Person.validates_absence_of :karma
+    p = Person.new
+    p.karma = "good"
+    assert p.invalid?
+    assert_equal ["must be blank"], p.errors[:karma]
+    p.karma = nil
+    assert p.valid?
+  end
+
+  def test_validates_absence_of_for_ruby_class_with_custom_reader
+    CustomReader.validates_absence_of :karma
+    p = CustomReader.new
+    p[:karma] = "excellent"
+    assert p.invalid?
+    assert_equal ["must be blank"], p.errors[:karma]
+    p[:karma] = ""
+    assert p.valid?
+  end
+end