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/validators/length_validator.rb

@@ -0,0 +1,134 @@
+# 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 LengthValidator < GenericValidator
+
+      def initialize(field_name, options)
+        super
+        @field_name = field_name
+        @options = options
+
+        @min = options[:minimum] || options[:min]
+        @max = options[:maximum] || options[:max]
+        @equal = options[:is] || options[:equals]
+        @range = options[:within] || options[:in]
+
+        @validation_method ||= :range if @range
+        @validation_method ||= :min if @min && @max.nil?
+        @validation_method ||= :max if @max && @min.nil?
+        @validation_method ||= :equals unless @equal.nil?
+      end
+
+      def call(target)
+        field_value = target.validation_property_value(field_name)
+        return true if @options[:allow_nil] && field_value.nil?
+
+        field_value = '' if field_value.nil?
+
+        # XXX: HACK seems hacky to do this on every validation, probably should
+        #      do this elsewhere?
+        field = Extlib::Inflection.humanize(field_name)
+        min = @range ? @range.min : @min
+        max = @range ? @range.max : @max
+        equal = @equal
+
+        case @validation_method
+        when :range then
+          unless valid = @range.include?(field_value.size)
+            error_message = ValidationErrors.default_error_message(:length_between, field, min, max)
+          end
+        when :min then
+          unless valid = field_value.size >= min
+            error_message = ValidationErrors.default_error_message(:too_short, field, min)
+          end
+        when :max then
+          unless valid = field_value.size <= max
+            error_message = ValidationErrors.default_error_message(:too_long, field, max)
+          end
+        when :equals then
+          unless valid = field_value.size == equal
+            error_message = ValidationErrors.default_error_message(:wrong_length, field, equal)
+          end
+        end
+
+        error_message = @options[:message] || error_message
+
+        add_error(target, error_message, field_name) unless valid
+
+        return valid
+      end
+
+    end # class LengthValidator
+
+    module ValidatesLength
+
+      # Validates that the length of the attribute is equal to, less than,
+      # greater than or within a certain range (depending upon the options
+      # you specify).
+      #
+      # @option :allow_nil<Boolean> true/false (default is true)
+      # @option :minimum    ensures that the attribute's length is greater than
+      #   or equal to the supplied value
+      # @option :min        alias for :minimum
+      # @option :maximum    ensures the attribute's length is less than or equal
+      #   to the supplied value
+      # @option :max        alias for :maximum
+      # @option :equals     ensures the attribute's length is equal to the
+      #   supplied value
+      # @option :is         alias for :equals
+      # @option :in<Range>  given a Range, ensures that the attributes length is
+      #   include?'ed in the Range
+      # @option :within<Range>  alias for :in
+      #
+      # @example [Usage]
+      #
+      #   class Page
+      #
+      #     property high, Integer
+      #     property low, Integer
+      #     property just_right, Integer
+      #
+      #     validates_length :high, :min => 100000000000
+      #     validates_length :low, :equals => 0
+      #     validates_length :just_right, :within => 1..10
+      #
+      #     # a call to valid? will return false unless:
+      #     # high is greater than or equal to 100000000000
+      #     # low is equal to 0
+      #     # just_right is between 1 and 10 (inclusive of both 1 and 10)
+      #
+      def validates_length(*fields)
+        opts = opts_from_validator_args(fields)
+        add_validator_to_context(opts, fields, CouchRest::Validation::LengthValidator)
+      end
+
+    end # module ValidatesLength
+  end # module Validation
+end # module CouchRest