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/numeric_validator.rb

@@ -0,0 +1,81 @@
+module CouchRest
+  module Validation
+
+    ##
+    #
+    # @author Guy van den Berg
+    # @since  0.9
+    class NumericValidator < GenericValidator
+
+      def initialize(field_name, options={})
+        super
+        @field_name, @options = field_name, options
+        @options[:integer_only] = false unless @options.has_key?(:integer_only)
+      end
+
+      def call(target)
+        value = target.send(field_name)
+        return true if @options[:allow_nil] && value.nil?
+
+        value = value.kind_of?(BigDecimal) ? value.to_s('F') : value.to_s
+
+        error_message = @options[:message]
+        precision     = @options[:precision]
+        scale         = @options[:scale]
+
+        if @options[:integer_only]
+          return true if value =~ /\A[+-]?\d+\z/
+          error_message ||= ValidationErrors.default_error_message(:not_an_integer, field_name)
+        else
+          # FIXME: if precision and scale are not specified, can we assume that it is an integer?
+          #        probably not, as floating point numbers don't have hard
+          #        defined scale. the scale floats with the length of the
+          #        integral and precision. Ie. if precision = 10 and integral
+          #        portion of the number is 9834 (4 digits), the max scale will
+          #        be 6 (10 - 4). But if the integral length is 1, max scale
+          #        will be (10 - 1) = 9, so 1.234567890.
+          if precision && scale
+            #handles both Float when it has scale specified and BigDecimal
+            if precision > scale && scale > 0
+              return true if value =~ /\A[+-]?(?:\d{1,#{precision - scale}}|\d{0,#{precision - scale}}\.\d{1,#{scale}})\z/
+            elsif precision > scale && scale == 0
+              return true if value =~ /\A[+-]?(?:\d{1,#{precision}}(?:\.0)?)\z/
+            elsif precision == scale
+              return true if value =~ /\A[+-]?(?:0(?:\.\d{1,#{scale}})?)\z/
+            else
+              raise ArgumentError, "Invalid precision #{precision.inspect} and scale #{scale.inspect} for #{field_name} (value: #{value.inspect} #{value.class})"
+            end
+          elsif precision && scale.nil?
+            # for floats, if scale is not set
+
+            #total number of digits is less or equal precision
+            return true if value.gsub(/[^\d]/, '').length <= precision
+
+            #number of digits before decimal == precision, and the number is x.0. same as scale = 0
+            return true if value =~ /\A[+-]?(?:\d{1,#{precision}}(?:\.0)?)\z/
+          else
+            return true if value =~ /\A[+-]?(?:\d+|\d*\.\d+)\z/
+          end
+          error_message ||= ValidationErrors.default_error_message(:not_a_number, field_name)
+        end
+
+        add_error(target, error_message, field_name)
+
+        # TODO: check the gt, gte, lt, lte, and eq options
+
+        return false
+      end
+    end # class NumericValidator
+
+    module ValidatesIsNumber
+
+      # Validate whether a field is numeric
+      #
+      def validates_is_number(*fields)
+        opts = opts_from_validator_args(fields)
+        add_validator_to_context(opts, fields, CouchRest::Validation::NumericValidator)
+      end
+
+    end # module ValidatesIsNumber
+  end # module Validation
+end # module CouchRest