Started added a validation mixin

Usage:
  class Invoice < CouchRest::ExtendedDocument
    include CouchRest::Validation

    property :client_name
    property :employee_name
    property :location

    # Validation
    validates_present :client_name, :employee_name
    validates_present :location, :message => "Hey stupid!, you forgot the location"

  end

lib/couchrest/validation/validation_errors.rb

@@ -0,0 +1,118 @@
+# Extracted from dm-validations 0.9.10
+#
+# Copyright (c) 2007 Guy van den Berg
+# 
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+# 
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+# 
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+module CouchRest
+  module Validation
+
+    ##
+    #
+    # @author Guy van den Berg
+    # @since  0.9
+    class ValidationErrors
+
+      include Enumerable
+
+      @@default_error_messages = {
+        :absent => '%s must be absent',
+        :inclusion => '%s must be one of [%s]',
+        :invalid => '%s has an invalid format',
+        :confirmation => '%s does not match the confirmation',
+        :accepted  => "%s is not accepted",
+        :nil => '%s must not be nil',
+        :blank => '%s must not be blank',
+        :length_between => '%s must be between %s and %s characters long',
+        :too_long => '%s must be less than %s characters long',
+        :too_short => '%s must be more than %s characters long',
+        :wrong_length => '%s must be %s characters long',
+        :taken => '%s is already taken',
+        :not_a_number => '%s must be a number',
+        :not_an_integer => '%s must be an integer',
+        :greater_than => '%s must be greater than %s',
+        :greater_than_or_equal_to => "%s must be greater than or equal to %s",
+        :equal_to => "%s must be equal to %s",
+        :less_than => '%s must be less than %s',
+        :less_than_or_equal_to => "%s must be less than or equal to %s",
+        :value_between => '%s must be between %s and %s',
+        :primitive => '%s must be of type %s'
+      }
+
+      # Holds a hash with all the default error messages that can be replaced by your own copy or localizations.
+      cattr_writer :default_error_messages
+
+      def self.default_error_message(key, field, *values)
+        field = Extlib::Inflection.humanize(field)
+        @@default_error_messages[key] % [field, *values].flatten
+      end
+
+      # Clear existing validation errors.
+      def clear!
+        errors.clear
+      end
+
+      # Add a validation error. Use the field_name :general if the errors does
+      # not apply to a specific field of the Resource.
+      #
+      # @param <Symbol> field_name the name of the field that caused the error
+      # @param <String> message    the message to add
+      def add(field_name, message)
+        (errors[field_name] ||= []) << message
+      end
+
+      # Collect all errors into a single list.
+      def full_messages
+        errors.inject([]) do |list, pair|
+          list += pair.last
+        end
+      end
+
+      # Return validation errors for a particular field_name.
+      #
+      # @param <Symbol> field_name the name of the field you want an error for
+      def on(field_name)
+        errors_for_field = errors[field_name]
+        errors_for_field.blank? ? nil : errors_for_field
+      end
+
+      def each
+        errors.map.each do |k, v|
+          next if v.blank?
+          yield(v)
+        end
+      end
+
+      def empty?
+        entries.empty?
+      end
+
+      def method_missing(meth, *args, &block)
+        errors.send(meth, *args, &block)
+      end
+
+      private
+      def errors
+        @errors ||= {}
+      end
+
+    end # class ValidationErrors
+  end # module Validation
+end # module CouchRest