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.

1 files changed, 31 insertions, 0 deletions

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