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/generic_validator.rb

@@ -0,0 +1,120 @@
+# -*- coding: utf-8 -*-
+
+# 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
+
+    # All validators extend this base class. Validators must:
+    #
+    # * Implement the initialize method to capture its parameters, also calling
+    #   super to have this parent class capture the optional, general :if and
+    #   :unless parameters.
+    # * Implement the call method, returning true or false. The call method
+    #   provides the validation logic.
+    #
+    # @author Guy van den Berg
+    class GenericValidator
+
+      attr_accessor :if_clause, :unless_clause
+      attr_reader :field_name
+
+      # Construct a validator. Capture the :if and :unless clauses when present.
+      #
+      # @param field<String, Symbol> The property specified for validation
+      #
+      # @option :if<Symbol, Proc>   The name of a method or a Proc to call to
+      #                     determine if the validation should occur.
+      # @option :unless<Symbol, Proc> The name of a method or a Proc to call to
+      #                         determine if the validation should not occur
+      # All additional key/value pairs are passed through to the validator
+      # that is sub-classing this GenericValidator
+      #
+      def initialize(field, opts = {})
+        @if_clause     = opts.delete(:if)
+        @unless_clause = opts.delete(:unless)
+      end
+
+      # Add an error message to a target resource. If the error corresponds to a
+      # specific field of the resource, add it to that field, otherwise add it
+      # as a :general message.
+      #
+      # @param <Object> target the resource that has the error
+      # @param <String> message the message to add
+      # @param <Symbol> field_name the name of the field that caused the error
+      #
+      # TODO - should the field_name for a general message be :default???
+      #
+      def add_error(target, message, field_name = :general)
+        target.errors.add(field_name, message)
+      end
+
+      # Call the validator. "call" is used so the operation is BoundMethod and
+      # Block compatible. This must be implemented in all concrete classes.
+      #
+      # @param <Object> target  the resource that the validator must be called
+      #                         against
+      # @return <Boolean> true if valid, otherwise false
+      def call(target)
+        raise NotImplementedError, "CouchRest::Validation::GenericValidator::call must be overriden in a subclass"
+      end
+
+      # Determines if this validator should be run against the
+      # target by evaluating the :if and :unless clauses
+      # optionally passed while specifying any validator.
+      #
+      # @param <Object> target the resource that we check against
+      # @return <Boolean> true if should be run, otherwise false
+      def execute?(target)
+        if unless_clause = self.unless_clause
+          if unless_clause.is_a?(Symbol)
+            return false if target.send(unless_clause)
+          elsif unless_clause.respond_to?(:call)
+            return false if unless_clause.call(target)
+          end
+        end
+
+        if if_clause = self.if_clause
+          if if_clause.is_a?(Symbol)
+            return target.send(if_clause)
+          elsif if_clause.respond_to?(:call)
+            return if_clause.call(target)
+          end
+        end
+
+        true
+      end
+
+      def ==(other)
+        self.class == other.class &&
+        self.field_name == other.field_name &&
+        self.class == other.class &&
+        self.if_clause == other.if_clause &&
+        self.unless_clause == other.unless_clause &&
+        self.instance_variable_get(:@options) == other.instance_variable_get(:@options)
+      end
+
+    end # class GenericValidator
+  end # module Validation
+end # module CouchRest