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/method_validator.rb

@@ -0,0 +1,66 @@
+module CouchRest
+  module Validation
+
+    ##
+    #
+    # @author Guy van den Berg
+    # @since  0.9
+    class MethodValidator < GenericValidator
+
+      def initialize(field_name, options={})
+        super
+        @field_name, @options = field_name, options.clone
+        @options[:method] = @field_name unless @options.has_key?(:method)
+      end
+
+      def call(target)
+        result, message = target.send(@options[:method])
+        add_error(target, message, field_name) unless result
+        result
+      end
+
+      def ==(other)
+        @options[:method] == other.instance_variable_get(:@options)[:method] && super
+      end
+    end # class MethodValidator
+
+    module ValidatesWithMethod
+
+      ##
+      # Validate using the given method. The method given needs to return:
+      # [result::<Boolean>, Error Message::<String>]
+      #
+      # @example [Usage]
+      #
+      #   class Page
+      #
+      #     property :zip_code, String
+      #
+      #     validates_with_method :in_the_right_location?
+      #
+      #     def in_the_right_location?
+      #       if @zip_code == "94301"
+      #         return true
+      #       else
+      #         return [false, "You're in the wrong zip code"]
+      #       end
+      #     end
+      #
+      #     # A call to valid? will return false and
+      #     # populate the object's errors with "You're in the
+      #     # wrong zip code" unless zip_code == "94301"
+      #
+      #     # You can also specify field:
+      #
+      #     validates_with_method :zip_code, :in_the_right_location?
+      #
+      #     # it will add returned error message to :zip_code field
+      #
+      def validates_with_method(*fields)
+        opts = opts_from_validator_args(fields)
+        add_validator_to_context(opts, fields, CouchRest::Validation::MethodValidator)
+      end
+
+    end # module ValidatesWithMethod
+  end # module Validation
+end # module CouchRest