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/required_field_validator.rb

@@ -0,0 +1,86 @@
+module CouchRest
+  module Validation
+
+    ##
+    #
+    # @author Guy van den Berg
+    # @since  0.9
+    class RequiredFieldValidator < GenericValidator
+
+      def initialize(field_name, options={})
+        super
+        @field_name, @options = field_name, options
+      end
+
+      def call(target)
+        value = target.validation_property_value(field_name)
+        property = target.validation_property(field_name)
+        return true if present?(value, property)
+
+        error_message = @options[:message] || default_error(property)
+        add_error(target, error_message, field_name)
+
+        false
+      end
+
+      protected
+
+      # Boolean property types are considered present if non-nil.
+      # Other property types are considered present if non-blank.
+      # Non-properties are considered present if non-blank.
+      def present?(value, property)
+        boolean_type?(property) ? !value.nil? : !value.blank?
+      end
+
+      def default_error(property)
+        actual = boolean_type?(property) ? :nil : :blank
+        ValidationErrors.default_error_message(actual, field_name)
+      end
+
+      # Is +property+ a boolean property?
+      #
+      # Returns true for Boolean, ParanoidBoolean, TrueClass, etc. properties.
+      # Returns false for other property types.
+      # Returns false for non-properties.
+      def boolean_type?(property)
+        property ? property.primitive == TrueClass : false
+      end
+
+    end # class RequiredFieldValidator
+
+    module ValidatesPresent
+
+      ##
+      # Validates that the specified attribute is present.
+      #
+      # For most property types "being present" is the same as being "not
+      # blank" as determined by the attribute's #blank? method. However, in
+      # the case of Boolean, "being present" means not nil; i.e. true or
+      # false.
+      #
+      # @note
+      #   dm-core's support lib adds the blank? method to many classes,
+      # @see lib/dm-core/support/blank.rb (dm-core) for more information.
+      #
+      # @example [Usage]
+      #
+      #   class Page
+      #
+      #     property :required_attribute, String
+      #     property :another_required, String
+      #     property :yet_again, String
+      #
+      #     validates_present :required_attribute
+      #     validates_present :another_required, :yet_again
+      #
+      #     # a call to valid? will return false unless
+      #     # all three attributes are !blank?
+      #   end
+      def validates_present(*fields)
+        opts = opts_from_validator_args(fields)
+        add_validator_to_context(opts, fields, CouchRest::Validation::RequiredFieldValidator)
+      end
+
+    end # module ValidatesPresent
+  end # module Validation
+end # module CouchRest