Move all the Active Record validations to Active Model

14 files changed, 408 insertions, 214 deletions

diff --git a/activemodel/lib/active_model.rb b/activemodel/lib/active_model.rb
index 4ed7b0889d..e777351cea 100644
--- a/activemodel/lib/active_model.rb
+++ b/activemodel/lib/active_model.rb
@@ -1,5 +1,43 @@
-require 'active_model/observing'
-# disabled until they're tested
-# require 'active_model/callbacks'
-# require 'active_model/validations'
-require 'active_model/base'
\ No newline at end of file
+#--
+# Copyright (c) 2004-2009 David Heinemeier Hansson
+#
+# 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.
+#++
+
+begin
+  require 'active_support'
+rescue LoadError
+  activesupport_path = "#{File.dirname(__FILE__)}/../../activesupport/lib"
+  if File.directory?(activesupport_path)
+    $:.unshift activesupport_path
+    require 'active_support'
+  end
+end
+
+module ActiveModel
+  def self.load_all!
+    [Base]
+  end
+
+  autoload :Base, 'active_model/base'
+  autoload :Validations, 'active_model/validations'
+  autoload :Errors, 'active_model/errors'
+  autoload :DeprecatedErrorMethods, 'active_model/deprecated_error_methods'
+end
diff --git a/activemodel/lib/active_model/deprecated_error_methods.rb b/activemodel/lib/active_model/deprecated_error_methods.rb
index e0cbd9ba29..b0d7023046 100644
--- a/activemodel/lib/active_model/deprecated_error_methods.rb
+++ b/activemodel/lib/active_model/deprecated_error_methods.rb
@@ -1,37 +1,28 @@
 module ActiveModel
   module DeprecatedErrorMethods
     def on(attribute)
-      ActiveSupport::Deprecation.warn "Errors#on have been deprecated, use Errors#[] instead"
-      self[attribute]
+      # ActiveSupport::Deprecation.warn "Errors#on have been deprecated, use Errors#[] instead"
+      errors = self[attribute]
+      errors.size < 2 ? errors.first : errors
     end
 
     def on_base
-      ActiveSupport::Deprecation.warn "Errors#on_base have been deprecated, use Errors#[:base] instead"
+      # ActiveSupport::Deprecation.warn "Errors#on_base have been deprecated, use Errors#[:base] instead"
       on(:base)
     end
 
-    def add(attribute, msg = Errors.default_error_messages[:invalid])
-      ActiveSupport::Deprecation.warn "Errors#add(attribute, msg) has been deprecated, use Errors#[attribute] << msg instead"
-      self[attribute] << msg
-    end
-
     def add_to_base(msg)
-      ActiveSupport::Deprecation.warn "Errors#add_to_base(msg) has been deprecated, use Errors#[:base] << msg instead"
+      # ActiveSupport::Deprecation.warn "Errors#add_to_base(msg) has been deprecated, use Errors#[:base] << msg instead"
       self[:base] << msg
     end
   
     def invalid?(attribute)
-      ActiveSupport::Deprecation.warn "Errors#invalid?(attribute) has been deprecated, use Errors#[attribute].any? instead"
+      # ActiveSupport::Deprecation.warn "Errors#invalid?(attribute) has been deprecated, use Errors#[attribute].any? instead"
       self[attribute].any?
     end
 
-    def full_messages
-      ActiveSupport::Deprecation.warn "Errors#full_messages has been deprecated, use Errors#to_a instead"
-      to_a
-    end
-
     def each_full
-      ActiveSupport::Deprecation.warn "Errors#each_full has been deprecated, use Errors#to_a.each instead"
+      # ActiveSupport::Deprecation.warn "Errors#each_full has been deprecated, use Errors#to_a.each instead"
       to_a.each { |error| yield error }
     end
   end
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index bcf0810290..ad7ed723a1 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -1,40 +1,27 @@
 module ActiveModel
   class Errors < Hash
     include DeprecatedErrorMethods
-    
-    @@default_error_messages = {
-      :inclusion                => "is not included in the list",
-      :exclusion                => "is reserved",
-      :invalid                  => "is invalid",
-      :confirmation             => "doesn't match confirmation",
-      :accepted                 => "must be accepted",
-      :empty                    => "can't be empty",
-      :blank                    => "can't be blank",
-      :too_long                 => "is too long (maximum is %d characters)",
-      :too_short                => "is too short (minimum is %d characters)",
-      :wrong_length             => "is the wrong length (should be %d characters)",
-      :taken                    => "has already been taken",
-      :not_a_number             => "is not a number",
-      :greater_than             => "must be greater than %d",
-      :greater_than_or_equal_to => "must be greater than or equal to %d",
-      :equal_to                 => "must be equal to %d",
-      :less_than                => "must be less than %d",
-      :less_than_or_equal_to    => "must be less than or equal to %d",
-      :odd                      => "must be odd",
-      :even                     => "must be even"
-    }
-  
-    ##
-    # :singleton-method:
-    # Holds a hash with all the default error messages that can be replaced by your own copy or localizations.
-    cattr_accessor :default_error_messages
+
+    class << self
+      def default_error_messages
+        message = "Errors.default_error_messages has been deprecated. Please use I18n.translate('activerecord.errors.messages')."
+        ActiveSupport::Deprecation.warn(message)
+
+        I18n.translate 'activerecord.errors.messages'
+      end
+    end
+
+    def initialize(base)
+      @base = base
+      super()
+    end
 
     alias_method :get, :[]
     alias_method :set, :[]=
 
     def [](attribute)
       if errors = get(attribute.to_sym)
-        errors.size == 1 ? errors.first : errors
+        errors
       else
         set(attribute.to_sym, [])
       end
@@ -55,17 +42,11 @@ module ActiveModel
     end
 
     def to_a
-      inject([]) do |errors_with_attributes, (attribute, errors)|
-        if error.blank?
-          errors_with_attributes
-        else
-          if attr == :base
-            errors_with_attributes << error
-          else
-            errors_with_attributes << (attribute.to_s.humanize + " " + error)
-          end
-        end
-      end
+      full_messages
+    end
+
+    def count
+      to_a.size
     end
 
     def to_xml(options={})
@@ -78,5 +59,106 @@ module ActiveModel
         to_a.each { |error| e.error(error) }
       end
     end
+
+    # Adds an error message (+messsage+) to the +attribute+, which will be returned on a call to <tt>on(attribute)</tt>
+    # for the same attribute and ensure that this error object returns false when asked if <tt>empty?</tt>. More than one
+    # error can be added to the same +attribute+ in which case an array will be returned on a call to <tt>on(attribute)</tt>.
+    # If no +messsage+ is supplied, :invalid is assumed.
+    # If +message+ is a Symbol, it will be translated, using the appropriate scope (see translate_error).
+    def add(attribute, message = nil, options = {})
+      message ||= :invalid
+      message = generate_message(attribute, message, options) if message.is_a?(Symbol)
+      self[attribute] << message
+    end
+
+    # Will add an error message to each of the attributes in +attributes+ that is empty.
+    def add_on_empty(attributes, custom_message = nil)
+      [attributes].flatten.each do |attribute|
+        value = @base.respond_to?(attribute.to_s) ? @base.send(attribute.to_s) : @base[attribute.to_s]
+        is_empty = value.respond_to?(:empty?) ? value.empty? : false
+        add(attribute, :empty, :default => custom_message) unless !value.nil? && !is_empty
+      end
+    end
+
+    # Will add an error message to each of the attributes in +attributes+ that is blank (using Object#blank?).
+    def add_on_blank(attributes, custom_message = nil)
+      [attributes].flatten.each do |attribute|
+        value = @base.respond_to?(attribute.to_sym) ? @base.send(attribute.to_sym) : @base[attribute.to_sym]
+        add(attribute, :blank, :default => custom_message) if value.blank?
+      end
+    end
+
+    # Returns all the full error messages in an array.
+    #
+    #   class Company < ActiveRecord::Base
+    #     validates_presence_of :name, :address, :email
+    #     validates_length_of :name, :in => 5..30
+    #   end
+    #
+    #   company = Company.create(:address => '123 First St.')
+    #   company.errors.full_messages # =>
+    #     ["Name is too short (minimum is 5 characters)", "Name can't be blank", "Address can't be blank"]
+    def full_messages(options = {})
+      full_messages = []
+
+      each do |attribute, messages|
+        next if messages.empty?
+
+        if attribute == :base
+          messages.each {|m| full_messages << m }
+        else
+          attr_name = @base.class.human_attribute_name(attribute.to_s)
+          prefix = attr_name + I18n.t('activerecord.errors.format.separator', :default => ' ')
+          messages.each do |m|
+            full_messages <<  "#{prefix}#{m}"
+          end
+        end
+      end
+
+      full_messages
+    end
+
+    # Translates an error message in it's default scope (<tt>activerecord.errrors.messages</tt>).
+    # Error messages are first looked up in <tt>models.MODEL.attributes.ATTRIBUTE.MESSAGE</tt>, if it's not there, 
+    # it's looked up in <tt>models.MODEL.MESSAGE</tt> and if that is not there it returns the translation of the 
+    # default message (e.g. <tt>activerecord.errors.messages.MESSAGE</tt>). The translated model name, 
+    # translated attribute name and the value are available for interpolation.
+    #
+    # When using inheritence in your models, it will check all the inherited models too, but only if the model itself
+    # hasn't been found. Say you have <tt>class Admin < User; end</tt> and you wanted the translation for the <tt>:blank</tt>
+    # error +message+ for the <tt>title</tt> +attribute+, it looks for these translations:
+    # 
+    # <ol>
+    # <li><tt>activerecord.errors.models.admin.attributes.title.blank</tt></li>
+    # <li><tt>activerecord.errors.models.admin.blank</tt></li>
+    # <li><tt>activerecord.errors.models.user.attributes.title.blank</tt></li>
+    # <li><tt>activerecord.errors.models.user.blank</tt></li>
+    # <li><tt>activerecord.errors.messages.blank</tt></li>
+    # <li>any default you provided through the +options+ hash (in the activerecord.errors scope)</li>
+    # </ol>
+    def generate_message(attribute, message = :invalid, options = {})
+      message, options[:default] = options[:default], message if options[:default].is_a?(Symbol)
+
+      defaults = @base.class.self_and_descendants_from_active_record.map do |klass|
+        [ :"models.#{klass.name.underscore}.attributes.#{attribute}.#{message}", 
+          :"models.#{klass.name.underscore}.#{message}" ]
+      end
+      
+      defaults << options.delete(:default)
+      defaults = defaults.compact.flatten << :"messages.#{message}"
+
+      key = defaults.shift
+      value = @base.respond_to?(attribute) ? @base.send(attribute) : nil
+
+      options = { :default => defaults,
+        :model => @base.class.human_name,
+        :attribute => @base.class.human_attribute_name(attribute.to_s),
+        :value => value,
+        :scope => [:activerecord, :errors]
+      }.merge(options)
+
+      I18n.translate(key, options)
+    end
+
   end
 end
\ No newline at end of file
diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index 460d2d82e5..3ea17381c9 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -38,7 +38,7 @@ module ActiveModel
       #   end
       #
       # This usage applies to +validate_on_create+ and +validate_on_update as well+.
-      #
+
       # Validates each attribute against a block.
       #
       #   class Person < ActiveRecord::Base
@@ -48,7 +48,7 @@ module ActiveModel
       #   end
       #
       # Options:
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
       # * <tt>:allow_nil</tt> - Skip validation if attribute is +nil+.
       # * <tt>:allow_blank</tt> - Skip validation if attribute is blank.
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
@@ -83,7 +83,7 @@ module ActiveModel
 
     # Returns the Errors object that holds all information about attribute error messages.
     def errors
-      @errors ||= Errors.new
+      @errors ||= Errors.new(self)
     end
 
     # Runs all the specified validations and returns true if no errors were added otherwise false.
@@ -92,35 +92,52 @@ module ActiveModel
 
       run_callbacks(:validate)
       
-      if responds_to?(:validate)
-        ActiveSupport::Deprecations.warn "Base#validate has been deprecated, please use Base.validate :method instead"
+      if respond_to?(:validate)
+        # ActiveSupport::Deprecation.warn "Base#validate has been deprecated, please use Base.validate :method instead"
         validate
       end
 
       if new_record?
         run_callbacks(:validate_on_create)
 
-        if responds_to?(:validate_on_create)
-          ActiveSupport::Deprecations.warn(
-            "Base#validate_on_create has been deprecated, please use Base.validate_on_create :method instead")
+        if respond_to?(:validate_on_create)
+          # ActiveSupport::Deprecation.warn "Base#validate_on_create has been deprecated, please use Base.validate_on_create :method instead"
           validate_on_create
         end
       else
         run_callbacks(:validate_on_update)
 
-        if responds_to?(:validate_on_update)
-          ActiveSupport::Deprecations.warn(
-            "Base#validate_on_update has been deprecated, please use Base.validate_on_update :method instead")
+        if respond_to?(:validate_on_update)
+          # ActiveSupport::Deprecation.warn "Base#validate_on_update has been deprecated, please use Base.validate_on_update :method instead"
           validate_on_update
         end
       end
 
       errors.empty?
     end
+
+    # Performs the opposite of <tt>valid?</tt>. Returns true if errors were added, false otherwise.
+    def invalid?
+      !valid?
+    end
+
+    protected
+    
+    # Overwrite this method for validation checks on all saves and use <tt>Errors.add(field, msg)</tt> for invalid attributes.
+    def validate
+    end
+
+    # Overwrite this method for validation checks used only on creation.
+    def validate_on_create
+    end
+
+    # Overwrite this method for validation checks used only on updates.
+    def validate_on_update
+    end
   end
 end
 
 Dir[File.dirname(__FILE__) + "/validations/*.rb"].sort.each do |path|
   filename = File.basename(path)
   require "active_model/validations/#{filename}"
-end
\ No newline at end of file
+end
diff --git a/activemodel/lib/active_model/validations/acceptance.rb b/activemodel/lib/active_model/validations/acceptance.rb
index 9be7d51ffb..db1fd04bb5 100644
--- a/activemodel/lib/active_model/validations/acceptance.rb
+++ b/activemodel/lib/active_model/validations/acceptance.rb
@@ -8,16 +8,16 @@ module ActiveModel
       #     validates_acceptance_of :eula, :message => "must be abided"
       #   end
       #
-      # If the database column does not exist, the <tt>:terms_of_service</tt> attribute is entirely virtual. This check is
-      # performed only if <tt>:terms_of_service</tt> is not +nil+ and by default on save.
+      # If the database column does not exist, the +terms_of_service+ attribute is entirely virtual. This check is
+      # performed only if +terms_of_service+ is not +nil+ and by default on save.
       #
       # Configuration options:
-      # * <tt>:message</tt> - A custom error message (default is: "must be accepted")
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
-      # * <tt>:allow_nil</tt> - Skip validation if attribute is +nil+. (default is +true+)
+      # * <tt>:message</tt> - A custom error message (default is: "must be accepted").
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
+      # * <tt>:allow_nil</tt> - Skip validation if attribute is +nil+ (default is true).
       # * <tt>:accept</tt> - Specifies value that is considered accepted.  The default value is a string "1", which
       #   makes it easy to relate to an HTML checkbox. This should be set to +true+ if you are validating a database
-      #   column, since the attribute is typecasted from "1" to +true+ before validation.
+      #   column, since the attribute is typecast from "1" to +true+ before validation.
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -25,19 +25,22 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_acceptance_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:accepted], :on => :save, :allow_nil => true, :accept => "1" }
+        configuration = { :on => :save, :allow_nil => true, :accept => "1" }
         configuration.update(attr_names.extract_options!)
 
         db_cols = begin
           column_names
-        rescue ActiveRecord::StatementInvalid
+        rescue Exception # To ignore both statement and connection errors
           []
         end
+
         names = attr_names.reject { |name| db_cols.include?(name.to_s) }
         attr_accessor(*names)
 
         validates_each(attr_names,configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message]) unless value == configuration[:accept]
+          unless value == configuration[:accept]
+            record.errors.add(attr_name, :accepted, :default => configuration[:message]) 
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/associated.rb b/activemodel/lib/active_model/validations/associated.rb
index b2d78af580..ba4af5d66d 100644
--- a/activemodel/lib/active_model/validations/associated.rb
+++ b/activemodel/lib/active_model/validations/associated.rb
@@ -18,14 +18,14 @@ module ActiveModel
       #     validates_associated :book
       #   end
       #
-      # ...this would specify a circular dependency and cause infinite recursion.
+      # this would specify a circular dependency and cause infinite recursion.
       #
       # NOTE: This validation will not fail if the association hasn't been assigned. If you want to ensure that the association
       # is both present and guaranteed to be valid, you also need to use +validates_presence_of+.
       #
       # Configuration options:
       # * <tt>:message</tt> - A custom error message (default is: "is invalid")
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -33,12 +33,13 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_associated(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:invalid], :on => :save }
+        configuration = { :on => :save }
         configuration.update(attr_names.extract_options!)
 
         validates_each(attr_names, configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message]) unless
-            (value.is_a?(Array) ? value : [value]).inject(true) { |v, r| (r.nil? || r.valid?) && v }
+          unless (value.is_a?(Array) ? value : [value]).collect { |r| r.nil? || r.valid? }.all?
+            record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value)
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/confirmation.rb b/activemodel/lib/active_model/validations/confirmation.rb
index ba4a18adb7..5f18da58e3 100644
--- a/activemodel/lib/active_model/validations/confirmation.rb
+++ b/activemodel/lib/active_model/validations/confirmation.rb
@@ -21,8 +21,8 @@ module ActiveModel
       #   validates_presence_of :password_confirmation, :if => :password_changed?
       #
       # Configuration options:
-      # * <tt>:message</tt> - A custom error message (default is: "doesn't match confirmation")
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
+      # * <tt>:message</tt> - A custom error message (default is: "doesn't match confirmation").
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -30,13 +30,15 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_confirmation_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:confirmation], :on => :save }
+        configuration = { :on => :save }
         configuration.update(attr_names.extract_options!)
 
         attr_accessor(*(attr_names.map { |n| "#{n}_confirmation" }))
 
         validates_each(attr_names, configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message]) unless record.send("#{attr_name}_confirmation").nil? or value == record.send("#{attr_name}_confirmation")
+          unless record.send("#{attr_name}_confirmation").nil? or value == record.send("#{attr_name}_confirmation")
+            record.errors.add(attr_name, :confirmation, :default => configuration[:message]) 
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/exclusion.rb b/activemodel/lib/active_model/validations/exclusion.rb
index 1592000fc7..435ba5cab6 100644
--- a/activemodel/lib/active_model/validations/exclusion.rb
+++ b/activemodel/lib/active_model/validations/exclusion.rb
@@ -6,14 +6,14 @@ module ActiveModel
       #   class Person < ActiveRecord::Base
       #     validates_exclusion_of :username, :in => %w( admin superuser ), :message => "You don't belong here"
       #     validates_exclusion_of :age, :in => 30..60, :message => "This site is only for under 30 and over 60"
-      #     validates_exclusion_of :format, :in => %w( mov avi ), :message => "extension %s is not allowed"
+      #     validates_exclusion_of :format, :in => %w( mov avi ), :message => "extension {{value}} is not allowed"
       #   end
       #
       # Configuration options:
-      # * <tt>:in</tt> - An enumerable object of items that the value shouldn't be part of
-      # * <tt>:message</tt> - Specifies a custom error message (default is: "is reserved")
-      # * <tt>:allow_nil</tt> - If set to +true+, skips this validation if the attribute is +nil+ (default is: +false+)
-      # * <tt>:allow_blank</tt> - If set to +true+, skips this validation if the attribute is blank (default is: +false+)
+      # * <tt>:in</tt> - An enumerable object of items that the value shouldn't be part of.
+      # * <tt>:message</tt> - Specifies a custom error message (default is: "is reserved").
+      # * <tt>:allow_nil</tt> - If set to true, skips this validation if the attribute is +nil+ (default is +false+).
+      # * <tt>:allow_blank</tt> - If set to true, skips this validation if the attribute is blank (default is +false+).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -21,7 +21,7 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_exclusion_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:exclusion], :on => :save }
+        configuration = { :on => :save }
         configuration.update(attr_names.extract_options!)
 
         enum = configuration[:in] || configuration[:within]
@@ -29,7 +29,9 @@ module ActiveModel
         raise(ArgumentError, "An object with the method include? is required must be supplied as the :in option of the configuration hash") unless enum.respond_to?(:include?)
 
         validates_each(attr_names, configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message] % value) if enum.include?(value)
+          if enum.include?(value)
+            record.errors.add(attr_name, :exclusion, :default => configuration[:message], :value => value) 
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/format.rb b/activemodel/lib/active_model/validations/format.rb
index 1320ef646a..c71567b993 100644
--- a/activemodel/lib/active_model/validations/format.rb
+++ b/activemodel/lib/active_model/validations/format.rb
@@ -13,11 +13,11 @@ module ActiveModel
       # A regular expression must be provided or else an exception will be raised.
       #
       # Configuration options:
-      # * <tt>:message</tt> - A custom error message (default is: "is invalid")
-      # * <tt>:allow_nil</tt> - If set to +true+, skips this validation if the attribute is +nil+ (default is: +false+)
-      # * <tt>:allow_blank</tt> - If set to +true+, skips this validation if the attribute is blank (default is: +false+)
-      # * <tt>:with</tt> - The regular expression used to validate the format with (note: must be supplied!)
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
+      # * <tt>:message</tt> - A custom error message (default is: "is invalid").
+      # * <tt>:allow_nil</tt> - If set to true, skips this validation if the attribute is +nil+ (default is +false+).
+      # * <tt>:allow_blank</tt> - If set to true, skips this validation if the attribute is blank (default is +false+).
+      # * <tt>:with</tt> - The regular expression used to validate the format with (note: must be supplied!).
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -25,13 +25,15 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_format_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:invalid], :on => :save, :with => nil }
+        configuration = { :on => :save, :with => nil }
         configuration.update(attr_names.extract_options!)
 
         raise(ArgumentError, "A regular expression must be supplied as the :with option of the configuration hash") unless configuration[:with].is_a?(Regexp)
 
         validates_each(attr_names, configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message]) unless value.to_s =~ configuration[:with]
+          unless value.to_s =~ configuration[:with]
+            record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value) 
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/inclusion.rb b/activemodel/lib/active_model/validations/inclusion.rb
index f288810d90..0cb8e44770 100644
--- a/activemodel/lib/active_model/validations/inclusion.rb
+++ b/activemodel/lib/active_model/validations/inclusion.rb
@@ -6,14 +6,14 @@ module ActiveModel
       #   class Person < ActiveRecord::Base
       #     validates_inclusion_of :gender, :in => %w( m f )
       #     validates_inclusion_of :age, :in => 0..99
-      #     validates_inclusion_of :format, :in => %w( jpg gif png ), :message => "extension %s is not included in the list"
+      #     validates_inclusion_of :format, :in => %w( jpg gif png ), :message => "extension {{value}} is not included in the list"
       #   end
       #
       # Configuration options:
-      # * <tt>:in</tt> - An enumerable object of available items
-      # * <tt>:message</tt> - Specifies a custom error message (default is: "is not included in the list")
-      # * <tt>:allow_nil</tt> - If set to +true+, skips this validation if the attribute is null (default is: +false+)
-      # * <tt>:allow_blank</tt> - If set to +true+, skips this validation if the attribute is blank (default is: +false+)
+      # * <tt>:in</tt> - An enumerable object of available items.
+      # * <tt>:message</tt> - Specifies a custom error message (default is: "is not included in the list").
+      # * <tt>:allow_nil</tt> - If set to true, skips this validation if the attribute is +nil+ (default is +false+).
+      # * <tt>:allow_blank</tt> - If set to true, skips this validation if the attribute is blank (default is +false+).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
@@ -21,7 +21,7 @@ module ActiveModel
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       def validates_inclusion_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:inclusion], :on => :save }
+        configuration = { :on => :save }
         configuration.update(attr_names.extract_options!)
 
         enum = configuration[:in] || configuration[:within]
@@ -29,7 +29,9 @@ module ActiveModel
         raise(ArgumentError, "An object with the method include? is required must be supplied as the :in option of the configuration hash") unless enum.respond_to?(:include?)
 
         validates_each(attr_names, configuration) do |record, attr_name, value|
-          record.errors.add(attr_name, configuration[:message] % value) unless enum.include?(value)
+          unless enum.include?(value)
+            record.errors.add(attr_name, :inclusion, :default => configuration[:message], :value => value) 
+          end
         end
       end
     end
diff --git a/activemodel/lib/active_model/validations/length.rb b/activemodel/lib/active_model/validations/length.rb
index 673ad33974..f6bb1f6d70 100644
--- a/activemodel/lib/active_model/validations/length.rb
+++ b/activemodel/lib/active_model/validations/length.rb
@@ -2,44 +2,46 @@ module ActiveModel
   module Validations
     module ClassMethods
       ALL_RANGE_OPTIONS = [ :is, :within, :in, :minimum, :maximum ].freeze
-    
+
       # Validates that the specified attribute matches the length restrictions supplied. Only one option can be used at a time:
       #
       #   class Person < ActiveRecord::Base
-      #     validates_length_of :first_name, :maximum => 30
-      #     validates_length_of :last_name, :maximum => 30, :message => "less than %d if you don't mind"
+      #     validates_length_of :first_name, :maximum=>30
+      #     validates_length_of :last_name, :maximum=>30, :message=>"less than {{count}} if you don't mind"
       #     validates_length_of :fax, :in => 7..32, :allow_nil => true
       #     validates_length_of :phone, :in => 7..32, :allow_blank => true
       #     validates_length_of :user_name, :within => 6..20, :too_long => "pick a shorter name", :too_short => "pick a longer name"
-      #     validates_length_of :fav_bra_size, :minimum => 1, :too_short => "please enter at least %d character"
-      #     validates_length_of :smurf_leader, :is => 4, :message => "papa is spelled with %d characters... don't play me."
+      #     validates_length_of :fav_bra_size, :minimum => 1, :too_short => "please enter at least {{count}} character"
+      #     validates_length_of :smurf_leader, :is => 4, :message => "papa is spelled with {{count}} characters... don't play me."
+      #     validates_length_of :essay, :minimum => 100, :too_short => "Your essay must be at least {{count}} words."), :tokenizer => lambda {|str| str.scan(/\w+/) }
       #   end
       #
       # Configuration options:
-      # * <tt>:minimum</tt> - The minimum size of the attribute
-      # * <tt>:maximum</tt> - The maximum size of the attribute
-      # * <tt>:is</tt> - The exact size of the attribute
-      # * <tt>:within</tt> - A range specifying the minimum and maximum size of the attribute
-      # * <tt>:in</tt> - A synonym (or alias) for <tt>:within</tt>
+      # * <tt>:minimum</tt> - The minimum size of the attribute.
+      # * <tt>:maximum</tt> - The maximum size of the attribute.
+      # * <tt>:is</tt> - The exact size of the attribute.
+      # * <tt>:within</tt> - A range specifying the minimum and maximum size of the attribute.
+      # * <tt>:in</tt> - A synonym(or alias) for <tt>:within</tt>.
       # * <tt>:allow_nil</tt> - Attribute may be +nil+; skip validation.
       # * <tt>:allow_blank</tt> - Attribute may be blank; skip validation.
-      # * <tt>:too_long</tt> - The error message if the attribute goes over the maximum (default is: "is too long (maximum is %d characters)")
-      # * <tt>:too_short</tt> - The error message if the attribute goes under the minimum (default is: "is too short (min is %d characters)")
-      # * <tt>:wrong_length</tt> - The error message if using the <tt>:is</tt> method and the attribute is the wrong size (default is: "is the wrong length (should be %d characters)")
-      # * <tt>:message</tt> - The error message to use for a <tt>:minimum</tt>, <tt>:maximum</tt>, or <tt>:is</tt> violation.  An alias of the appropriate <tt>:too_long</tt>/<tt>too_short</tt>/<tt>wrong_length</tt> message
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
+      # * <tt>:too_long</tt> - The error message if the attribute goes over the maximum (default is: "is too long (maximum is {{count}} characters)").
+      # * <tt>:too_short</tt> - The error message if the attribute goes under the minimum (default is: "is too short (min is {{count}} characters)").
+      # * <tt>:wrong_length</tt> - The error message if using the <tt>:is</tt> method and the attribute is the wrong size (default is: "is the wrong length (should be {{count}} characters)").
+      # * <tt>:message</tt> - The error message to use for a <tt>:minimum</tt>, <tt>:maximum</tt>, or <tt>:is</tt> violation.  An alias of the appropriate <tt>too_long</tt>/<tt>too_short</tt>/<tt>wrong_length</tt> message.
+      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
+      # * <tt>:tokenizer</tt> - Specifies how to split up the attribute string. (e.g. <tt>:tokenizer => lambda {|str| str.scan(/\w+/)}</tt> to
+      #   count words as in above example.)
+      #   Defaults to <tt>lambda{ |value| value.split(//) }</tt> which counts individual characters.
       def validates_length_of(*attrs)
         # Merge given options with defaults.
         options = {
-          :too_long     => ActiveRecord::Errors.default_error_messages[:too_long],
-          :too_short    => ActiveRecord::Errors.default_error_messages[:too_short],
-          :wrong_length => ActiveRecord::Errors.default_error_messages[:wrong_length]
+          :tokenizer => lambda {|value| value.split(//)}
         }.merge(DEFAULT_VALIDATION_OPTIONS)
         options.update(attrs.extract_options!.symbolize_keys)
 
@@ -57,35 +59,33 @@ module ActiveModel
         # Get range option and value.
         option = range_options.first
         option_value = options[range_options.first]
+        key = {:is => :wrong_length, :minimum => :too_short, :maximum => :too_long}[option]
+        custom_message = options[:message] || options[key]
 
         case option
-          when :within, :in
-            raise ArgumentError, ":#{option} must be a Range" unless option_value.is_a?(Range)
-
-            too_short = options[:too_short] % option_value.begin
-            too_long  = options[:too_long]  % option_value.end
+        when :within, :in
+          raise ArgumentError, ":#{option} must be a Range" unless option_value.is_a?(Range)
 
-            validates_each(attrs, options) do |record, attr, value|
-              value = value.split(//) if value.kind_of?(String)
-              if value.nil? or value.size < option_value.begin
-                record.errors.add(attr, too_short)
-              elsif value.size > option_value.end
-                record.errors.add(attr, too_long)
-              end
+          validates_each(attrs, options) do |record, attr, value|
+            value = options[:tokenizer].call(value) if value.kind_of?(String)
+            if value.nil? or value.size < option_value.begin
+              record.errors.add(attr, :too_short, :default => custom_message || options[:too_short], :count => option_value.begin)
+            elsif value.size > option_value.end
+              record.errors.add(attr, :too_long, :default => custom_message || options[:too_long], :count => option_value.end)
             end
-          when :is, :minimum, :maximum
-            raise ArgumentError, ":#{option} must be a nonnegative Integer" unless option_value.is_a?(Integer) and option_value >= 0
-
-            # Declare different validations per option.
-            validity_checks = { :is => "==", :minimum => ">=", :maximum => "<=" }
-            message_options = { :is => :wrong_length, :minimum => :too_short, :maximum => :too_long }
+          end
+        when :is, :minimum, :maximum
+          raise ArgumentError, ":#{option} must be a nonnegative Integer" unless option_value.is_a?(Integer) and option_value >= 0
 
-            message = (options[:message] || options[message_options[option]]) % option_value
+          # Declare different validations per option.
+          validity_checks = { :is => "==", :minimum => ">=", :maximum => "<=" }
 
-            validates_each(attrs, options) do |record, attr, value|
-              value = value.split(//) if value.kind_of?(String)
-              record.errors.add(attr, message) unless !value.nil? and value.size.method(validity_checks[option])[option_value]
+          validates_each(attrs, options) do |record, attr, value|
+            value = options[:tokenizer].call(value) if value.kind_of?(String)
+            unless !value.nil? and value.size.method(validity_checks[option])[option_value]
+              record.errors.add(attr, key, :default => custom_message, :count => option_value) 
             end
+          end
         end
       end
 
diff --git a/activemodel/lib/active_model/validations/numericality.rb b/activemodel/lib/active_model/validations/numericality.rb
index 92ca5f4082..d8e3470304 100644
--- a/activemodel/lib/active_model/validations/numericality.rb
+++ b/activemodel/lib/active_model/validations/numericality.rb
@@ -5,10 +5,9 @@ module ActiveModel
                                   :equal_to => '==', :less_than => '<', :less_than_or_equal_to => '<=',
                                   :odd => 'odd?', :even => 'even?' }.freeze
 
-
       # Validates whether the value of the specified attribute is numeric by trying to convert it to
-      # a float with Kernel.Float (if <tt>integer</tt> is false) or applying it to the regular expression
-      # <tt>/\A[\+\-]?\d+\Z/</tt> (if <tt>integer</tt> is true).
+      # a float with Kernel.Float (if <tt>only_integer</tt> is false) or applying it to the regular expression
+      # <tt>/\A[\+\-]?\d+\Z/</tt> (if <tt>only_integer</tt> is set to true).
       #
       #   class Person < ActiveRecord::Base
       #     validates_numericality_of :value, :on => :create
@@ -50,15 +49,15 @@ module ActiveModel
 
           if configuration[:only_integer]
             unless raw_value.to_s =~ /\A[+-]?\d+\Z/
-              record.errors.add(attr_name, configuration[:message] || ActiveRecord::Errors.default_error_messages[:not_a_number])
+              record.errors.add(attr_name, :not_a_number, :value => raw_value, :default => configuration[:message])
               next
             end
             raw_value = raw_value.to_i
           else
-           begin
-              raw_value = Kernel.Float(raw_value.to_s)
+            begin
+              raw_value = Kernel.Float(raw_value)
             rescue ArgumentError, TypeError
-              record.errors.add(attr_name, configuration[:message] || ActiveRecord::Errors.default_error_messages[:not_a_number])
+              record.errors.add(attr_name, :not_a_number, :value => raw_value, :default => configuration[:message])
               next
             end
           end
@@ -66,11 +65,13 @@ module ActiveModel
           numericality_options.each do |option|
             case option
               when :odd, :even
-                record.errors.add(attr_name, configuration[:message] || ActiveRecord::Errors.default_error_messages[option]) unless raw_value.to_i.method(ALL_NUMERICALITY_CHECKS[option])[]
+                unless raw_value.to_i.method(ALL_NUMERICALITY_CHECKS[option])[]
+                  record.errors.add(attr_name, option, :value => raw_value, :default => configuration[:message]) 
+                end
               else
-                message = configuration[:message] || ActiveRecord::Errors.default_error_messages[option]
-                message = message % configuration[option] if configuration[option]
-                record.errors.add(attr_name, message) unless raw_value.method(ALL_NUMERICALITY_CHECKS[option])[configuration[option]]
+                unless raw_value.method(ALL_NUMERICALITY_CHECKS[option])[configuration[option]]
+                  record.errors.add(attr_name, option, :default => configuration[:message], :value => raw_value, :count => configuration[option])
+                end
             end
           end
         end
diff --git a/activemodel/lib/active_model/validations/presence.rb b/activemodel/lib/active_model/validations/presence.rb
index 62e466901b..4fd60621cd 100644
--- a/activemodel/lib/active_model/validations/presence.rb
+++ b/activemodel/lib/active_model/validations/presence.rb
@@ -7,28 +7,26 @@ module ActiveModel
       #     validates_presence_of :first_name
       #   end
       #
-      # The +first_name+ attribute must be in the object and it cannot be blank.
+      # The first_name attribute must be in the object and it cannot be blank.
       #
-      # If you want to validate the presence of a boolean field (where the real values are +true+ and +false+),
-      # you will want to use
+      # If you want to validate the presence of a boolean field (where the real values are true and false),
+      # you will want to use <tt>validates_inclusion_of :field_name, :in => [true, false]</tt>.
       #
-      #   validates_inclusion_of :field_name, :in => [true, false]
-      #
-      # This is due to the way Object#blank? handles boolean values:
-      #
-      #   false.blank? # => true
+      # This is due to the way Object#blank? handles boolean values: <tt>false.blank? # => true</tt>.
       #
       # Configuration options:
-      # * <tt>:message</tt> - A custom error message (default is: "can't be blank")
-      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
-      # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
-      #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
-      #   method, proc or string should return or evaluate to a true or false value.
-      # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine if the validation should
-      #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
-      #   method, proc or string should return or evaluate to a true or false value.
+      # * <tt>message</tt> - A custom error message (default is: "can't be blank").
+      # * <tt>on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, 
+      #   <tt>:update</tt>).
+      # * <tt>if</tt> - Specifies a method, proc or string to call to determine if the validation should
+      #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).
+      #   The method, proc or string should return or evaluate to a true or false value.
+      # * <tt>unless</tt> - Specifies a method, proc or string to call to determine if the validation should
+      #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).
+      #   The method, proc or string should return or evaluate to a true or false value.
+      #
       def validates_presence_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:blank], :on => :save }
+        configuration = { :on => :save }
         configuration.update(attr_names.extract_options!)
 
         # can't use validates_each here, because it cannot cope with nonexistent attributes,
diff --git a/activemodel/lib/active_model/validations/uniqueness.rb b/activemodel/lib/active_model/validations/uniqueness.rb
index 593d2cfd54..37beac863f 100644
--- a/activemodel/lib/active_model/validations/uniqueness.rb
+++ b/activemodel/lib/active_model/validations/uniqueness.rb
@@ -18,24 +18,84 @@ module ActiveModel
       # When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified
       # attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself.
       #
-      # Because this check is performed outside the database there is still a chance that duplicate values
-      # will be inserted in two parallel transactions.  To guarantee against this you should create a 
-      # unique index on the field. See +add_index+ for more information.
-      #
       # Configuration options:
-      # * <tt>:message</tt> - Specifies a custom error message (default is: "has already been taken")
+      # * <tt>:message</tt> - Specifies a custom error message (default is: "has already been taken").
       # * <tt>:scope</tt> - One or more columns by which to limit the scope of the uniqueness constraint.
-      # * <tt>:case_sensitive</tt> - Looks for an exact match.  Ignored by non-text columns (+true+ by default).
-      # * <tt>:allow_nil</tt> - If set to +true+, skips this validation if the attribute is +nil+ (default is: +false+)
-      # * <tt>:allow_blank</tt> - If set to +true+, skips this validation if the attribute is blank (default is: +false+)
+      # * <tt>:case_sensitive</tt> - Looks for an exact match. Ignored by non-text columns (+true+ by default).
+      # * <tt>:allow_nil</tt> - If set to true, skips this validation if the attribute is +nil+ (default is +false+).
+      # * <tt>:allow_blank</tt> - If set to true, skips this validation if the attribute is blank (default is +false+).
       # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
       # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine if the validation should
       #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
       #   method, proc or string should return or evaluate to a true or false value.
+      #
+      # === Concurrency and integrity
+      #
+      # Using this validation method in conjunction with ActiveRecord::Base#save
+      # does not guarantee the absence of duplicate record insertions, because
+      # uniqueness checks on the application level are inherently prone to race
+      # conditions. For example, suppose that two users try to post a Comment at
+      # the same time, and a Comment's title must be unique. At the database-level,
+      # the actions performed by these users could be interleaved in the following manner:
+      #
+      #               User 1                 |               User 2
+      #  ------------------------------------+--------------------------------------
+      #  # User 1 checks whether there's     |
+      #  # already a comment with the title  |
+      #  # 'My Post'. This is not the case.  |
+      #  SELECT * FROM comments              |
+      #  WHERE title = 'My Post'             |
+      #                                      |
+      #                                      | # User 2 does the same thing and also
+      #                                      | # infers that his title is unique.
+      #                                      | SELECT * FROM comments
+      #                                      | WHERE title = 'My Post'
+      #                                      |
+      #  # User 1 inserts his comment.       |
+      #  INSERT INTO comments                |
+      #  (title, content) VALUES             |
+      #  ('My Post', 'hi!')                  |
+      #                                      |
+      #                                      | # User 2 does the same thing.
+      #                                      | INSERT INTO comments
+      #                                      | (title, content) VALUES
+      #                                      | ('My Post', 'hello!')
+      #                                      |
+      #                                      | # ^^^^^^
+      #                                      | # Boom! We now have a duplicate
+      #                                      | # title!
+      #
+      # This could even happen if you use transactions with the 'serializable'
+      # isolation level. There are several ways to get around this problem:
+      # - By locking the database table before validating, and unlocking it after
+      #   saving. However, table locking is very expensive, and thus not
+      #   recommended.
+      # - By locking a lock file before validating, and unlocking it after saving.
+      #   This does not work if you've scaled your Rails application across
+      #   multiple web servers (because they cannot share lock files, or cannot
+      #   do that efficiently), and thus not recommended.
+      # - Creating a unique index on the field, by using
+      #   ActiveRecord::ConnectionAdapters::SchemaStatements#add_index. In the
+      #   rare case that a race condition occurs, the database will guarantee
+      #   the field's uniqueness.
+      #   
+      #   When the database catches such a duplicate insertion,
+      #   ActiveRecord::Base#save will raise an ActiveRecord::StatementInvalid
+      #   exception. You can either choose to let this error propagate (which
+      #   will result in the default Rails exception page being shown), or you
+      #   can catch it and restart the transaction (e.g. by telling the user
+      #   that the title already exists, and asking him to re-enter the title).
+      #   This technique is also known as optimistic concurrency control:
+      #   http://en.wikipedia.org/wiki/Optimistic_concurrency_control
+      #   
+      #   Active Record currently provides no way to distinguish unique
+      #   index constraint errors from other types of database errors, so you
+      #   will have to parse the (database-specific) exception message to detect
+      #   such a case.
       def validates_uniqueness_of(*attr_names)
-        configuration = { :message => ActiveRecord::Errors.default_error_messages[:taken] }
+        configuration = { :case_sensitive => true }
         configuration.update(attr_names.extract_options!)
 
         validates_each(attr_names,configuration) do |record, attr_name, value|
@@ -53,20 +113,31 @@ module ActiveModel
           # class (which has a database table to query from).
           finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }
 
-          if value.nil? || (configuration[:case_sensitive] || !finder_class.columns_hash[attr_name.to_s].text?)
-            condition_sql = "#{record.class.quoted_table_name}.#{attr_name} #{attribute_condition(value)}"
+          column = finder_class.columns_hash[attr_name.to_s]
+
+          if value.nil?
+            comparison_operator = "IS ?"
+          elsif column.text?
+            comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
+            value = column.limit ? value.to_s[0, column.limit] : value.to_s
+          else
+            comparison_operator = "= ?"
+          end
+
+          sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}"
+
+          if value.nil? || (configuration[:case_sensitive] || !column.text?)
+            condition_sql = "#{sql_attribute} #{comparison_operator}"
             condition_params = [value]
           else
-            # sqlite has case sensitive SELECT query, while MySQL/Postgresql don't.
-            # Hence, this is needed only for sqlite.
-            condition_sql = "LOWER(#{record.class.quoted_table_name}.#{attr_name}) #{attribute_condition(value)}"
-            condition_params = [value.downcase]
+            condition_sql = "LOWER(#{sql_attribute}) #{comparison_operator}"
+            condition_params = [value.mb_chars.downcase]
           end
 
           if scope = configuration[:scope]
             Array(scope).map do |scope_item|
               scope_value = record.send(scope_item)
-              condition_sql << " AND #{record.class.quoted_table_name}.#{scope_item} #{attribute_condition(scope_value)}"
+              condition_sql << " AND " << attribute_condition("#{record.class.quoted_table_name}.#{scope_item}", scope_value)
               condition_params << scope_value
             end
           end
@@ -76,26 +147,10 @@ module ActiveModel
             condition_params << record.send(:id)
           end
 
-          results = finder_class.with_exclusive_scope do
-            connection.select_all(
-              construct_finder_sql(
-                :select     => attr_name,
-                :from       => finder_class.quoted_table_name,
-                :conditions => [condition_sql, *condition_params]
-              )
-            )
-          end
-
-          unless results.length.zero?
-            found = true
-
-            # As MySQL/Postgres don't have case sensitive SELECT queries, we try to find duplicate
-            # column in ruby when case sensitive option
-            if configuration[:case_sensitive] && finder_class.columns_hash[attr_name.to_s].text?
-              found = results.any? { |a| a[attr_name.to_s] == value }
+          finder_class.with_exclusive_scope do
+            if finder_class.exists?([condition_sql, *condition_params])
+              record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value)
             end
-
-            record.errors.add(attr_name, configuration[:message]) if found
           end
         end
       end