3 files changed, 45 insertions, 0 deletions
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 963e52bff3..b713e99e25 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -328,6 +328,19 @@ module ActiveModel
       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+.
     #
diff --git a/activemodel/lib/active_model/locale/en.yml b/activemodel/lib/active_model/locale/en.yml
index d17848c861..8ea34b84f5 100644
--- a/activemodel/lib/active_model/locale/en.yml
+++ b/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)"
diff --git a/activemodel/lib/active_model/validations/absence.rb b/activemodel/lib/active_model/validations/absence.rb
new file mode 100644
index 0000000000..6790554907
--- /dev/null
+++ b/activemodel/lib/active_model/validations/absence.rb
@@ -0,0 +1,31 @@
+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