module ActiveRecord
  # Raised by <tt>save!</tt> and <tt>create!</tt> when the record is invalid.  Use the
  # +record+ method to retrieve the record which did not validate.
  #   begin
  #     complex_operation_that_calls_save!_internally
  #   rescue ActiveRecord::RecordInvalid => invalid
  #     puts invalid.record.errors
  #   end
  class RecordInvalid < ActiveRecordError
    attr_reader :record
    def initialize(record)
      @record = record
      errors = @record.errors.full_messages.join(I18n.t('support.array.words_connector', :default => ', '))
      super(I18n.t('activerecord.errors.messages.record_invalid', :errors => errors))
    end
  end

  class Error
    attr_accessor :base, :attribute, :type, :message, :options

    def initialize(base, attribute, type = nil, options = {})
      self.base      = base
      self.attribute = attribute
      self.type      = type || :invalid
      self.options   = options
      self.message   = options.delete(:message) || self.type
    end

    def message
      # When type is a string, it means that we do not have to do a lookup, because
      # the user already sent the "final" message.
      type.is_a?(String) ? type : generate_message(default_options)
    end

    def full_message
      attribute.to_s == 'base' ? message : generate_full_message(default_options)
    end

    alias :to_s :message

    def value
      @base.respond_to?(attribute) ? @base.send(attribute) : nil
    end

    protected

      # 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(options = {})
        keys = @base.class.self_and_descendants_from_active_record.map do |klass|
          [ :"models.#{klass.name.underscore}.attributes.#{attribute}.#{@message}",
            :"models.#{klass.name.underscore}.#{@message}" ]
        end.flatten

        keys << options.delete(:default)
        keys << :"messages.#{@message}"
        keys << @message if @message.is_a?(String)
        keys << @type unless @type == @message
        keys.compact!

        options.merge!(:default => keys)
        I18n.translate(keys.shift, options)
      end

      # Wraps an error message into a full_message format.
      #
      # The default full_message format for any locale is <tt>"%{attribute} %{message}"</tt>.
      # One can specify locale specific default full_message format by storing it as a
      # translation for the key <tt>:"activerecord.errors.full_messages.format"</tt>.
      #
      # Additionally one can specify a validation specific error message format by
      # storing a translation for <tt>:"activerecord.errors.full_messages.[message_key]"</tt>.
      # E.g. the full_message format for any validation that uses :blank as a message
      # key (such as validates_presence_of) can be stored to <tt>:"activerecord.errors.full_messages.blank".</tt>
      #
      # Because the message key used by a validation can be overwritten on the
      # <tt>validates_*</tt> class macro level one can customize the full_message format for
      # any particular validation:
      #
      #   # app/models/article.rb
      #   class Article < ActiveRecord::Base
      #     validates_presence_of :title, :message => :"title.blank"
      #   end
      #
      #   # config/locales/en.yml
      #   en:
      #     activerecord:
      #       errors:
      #         full_messages:
      #           title:
      #             blank: This title is screwed!
      def generate_full_message(options = {})
        keys = [
          :"full_messages.#{@message}",
          :'full_messages.format',
          '%{attribute} %{message}'
        ]

        options.merge!(:default => keys, :message => self.message)
        I18n.translate(keys.shift, options)
      end

      # Return user options with default options.
      #
      def default_options
        options.reverse_merge :scope => [:activerecord, :errors],
                              :model => @base.class.human_name,
                              :attribute => @base.class.human_attribute_name(attribute.to_s),
                              :value => value
      end
  end

  # Active Record validation is reported to and from this object, which is used by Base#save to
  # determine whether the object is in a valid state to be saved. See usage example in Validations.
  class Errors
    include Enumerable

    class << self
      def default_error_messages
        ActiveSupport::Deprecation.warn("ActiveRecord::Errors.default_error_messages has been deprecated. Please use I18n.translate('activerecord.errors.messages').")
        I18n.translate 'activerecord.errors.messages'
      end
    end

    def initialize(base) # :nodoc:
      @base = base
      clear
    end

    # Adds an error to the base object instead of any particular attribute. This is used
    # to report errors that don't tie to any specific attribute, but rather to the object
    # as a whole. These error messages don't get prepended with any field name when iterating
    # with +each_full+, so they should be complete sentences.
    def add_to_base(msg)
      add(:base, msg)
    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 = {})
      options[:message] = options.delete(:default) if options[:default].is_a?(Symbol)
      error, message = message, nil if message.is_a?(Error)

      @errors[attribute.to_s] ||= []
      @errors[attribute.to_s] << (error || Error.new(@base, attribute, message, options))
    end

    # Will add an error message to each of the attributes in +attributes+ that is empty.
    def add_on_empty(attributes, custom_message = nil)
      for attr in [attributes].flatten
        value = @base.respond_to?(attr.to_s) ? @base.send(attr.to_s) : @base[attr.to_s]
        is_empty = value.respond_to?(:empty?) ? value.empty? : false
        add(attr, :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)
      for attr in [attributes].flatten
        value = @base.respond_to?(attr.to_s) ? @base.send(attr.to_s) : @base[attr.to_s]
        add(attr, :blank, :default => custom_message) if value.blank?
      end
    end

    # Returns true if the specified +attribute+ has errors associated with it.
    #
    #   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.invalid?(:name)      # => true
    #   company.errors.invalid?(:address)   # => false
    def invalid?(attribute)
      !@errors[attribute.to_s].nil?
    end

    # Returns +nil+, if no errors are associated with the specified +attribute+.
    # Returns the error message, if one error is associated with the specified +attribute+.
    # Returns an array of error messages, if more than one error is associated with the specified +attribute+.
    #
    #   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.on(:name)      # => ["is too short (minimum is 5 characters)", "can't be blank"]
    #   company.errors.on(:email)     # => "can't be blank"
    #   company.errors.on(:address)   # => nil
    def on(attribute)
      attribute = attribute.to_s
      return nil unless @errors.has_key?(attribute)
      errors = @errors[attribute].map(&:to_s)
      errors.size == 1 ? errors.first : errors
    end

    alias :[] :on

    # Returns errors assigned to the base object through +add_to_base+ according to the normal rules of <tt>on(attribute)</tt>.
    def on_base
      on(:base)
    end

    # Yields each attribute and associated message per error added.
    #
    #   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.each{|attr,msg| puts "#{attr} - #{msg}" }
    #   # => name - is too short (minimum is 5 characters)
    #   #    name - can't be blank
    #   #    address - can't be blank
    def each
      @errors.each_key { |attr| @errors[attr].each { |error| yield attr, error.message } }
    end

    # Yields each attribute and associated error per error added.
    #
    #   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.each_error{|attr,err| puts "#{attr} - #{err.type}" }
    #   # => name - :too_short
    #   #    name - :blank
    #   #    address - :blank
    def each_error
      @errors.each_key { |attr| @errors[attr].each { |error| yield attr, error } }
    end

    # Yields each full error message added. So <tt>Person.errors.add("first_name", "can't be empty")</tt> will be returned
    # through iteration as "First name can't be empty".
    #
    #   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.each_full{|msg| puts msg }
    #   # => Name is too short (minimum is 5 characters)
    #   #    Name can't be blank
    #   #    Address can't be blank
    def each_full
      full_messages.each { |msg| yield msg }
    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 = {})
      @errors.values.inject([]) do |full_messages, errors|
        full_messages + errors.map { |error| error.full_message }
      end
    end

    # Returns true if no errors have been added.
    def empty?
      @errors.empty?
    end

    # Removes all errors that have been added.
    def clear
      @errors = ActiveSupport::OrderedHash.new
    end

    # Returns the total number of errors added. Two errors added to the same attribute will be counted as such.
    def size
      @errors.values.inject(0) { |error_count, attribute| error_count + attribute.size }
    end

    alias_method :count, :size
    alias_method :length, :size

    # Returns an XML representation of this error object.
    #
    #   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.to_xml
    #   # =>  <?xml version="1.0" encoding="UTF-8"?>
    #   #     <errors>
    #   #       <error>Name is too short (minimum is 5 characters)</error>
    #   #       <error>Name can't be blank</error>
    #   #       <error>Address can't be blank</error>
    #   #     </errors>
    def to_xml(options={})
      options[:root] ||= "errors"
      options[:indent] ||= 2
      options[:builder] ||= Builder::XmlMarkup.new(:indent => options[:indent])

      options[:builder].instruct! unless options.delete(:skip_instruct)
      options[:builder].errors do |e|
        full_messages.each { |msg| e.error(msg) }
      end
    end

    def generate_message(attribute, message = :invalid, options = {})
      ActiveSupport::Deprecation.warn("ActiveRecord::Errors#generate_message has been deprecated. Please use ActiveRecord::Error.new().to_s.")
      Error.new(@base, attribute, message, options).to_s
    end
  end


  # Please do have a look at ActiveRecord::Validations::ClassMethods for a higher level of validations.
  #
  # Active Records implement validation by overwriting Base#validate (or the variations, +validate_on_create+ and
  # +validate_on_update+). Each of these methods can inspect the state of the object, which usually means ensuring
  # that a number of attributes have a certain value (such as not empty, within a given range, matching a certain regular expression).
  #
  # Example:
  #
  #   class Person < ActiveRecord::Base
  #     protected
  #       def validate
  #         errors.add_on_empty %w( first_name last_name )
  #         errors.add("phone_number", "has invalid format") unless phone_number =~ /[0-9]*/
  #       end
  #
  #       def validate_on_create # is only run the first time a new object is saved
  #         unless valid_discount?(membership_discount)
  #           errors.add("membership_discount", "has expired")
  #         end
  #       end
  #
  #       def validate_on_update
  #         errors.add_to_base("No changes have occurred") if unchanged_attributes?
  #       end
  #   end
  #
  #   person = Person.new("first_name" => "David", "phone_number" => "what?")
  #   person.save                         # => false (and doesn't do the save)
  #   person.errors.empty?                # => false
  #   person.errors.count                 # => 2
  #   person.errors.on "last_name"        # => "can't be empty"
  #   person.errors.on "phone_number"     # => "has invalid format"
  #   person.errors.each_full { |msg| puts msg }
  #                                       # => "Last name can't be empty\n" +
  #                                       #    "Phone number has invalid format"
  #
  #   person.attributes = { "last_name" => "Heinemeier", "phone_number" => "555-555" }
  #   person.save # => true (and person is now saved in the database)
  #
  # An Errors object is automatically created for every Active Record.
  module Validations
    VALIDATIONS = %w( validate validate_on_create validate_on_update )

    def self.included(base) # :nodoc:
      base.extend ClassMethods
      base.class_eval do
        alias_method_chain :save, :validation
        alias_method_chain :save!, :validation
      end

      base.send :include, ActiveSupport::Callbacks
      base.define_callbacks *VALIDATIONS
    end

    # Active Record classes can implement validations in several ways. The highest level, easiest to read,
    # and recommended approach is to use the declarative <tt>validates_..._of</tt> class methods (and
    # +validates_associated+) documented below. These are sufficient for most model validations.
    #
    # Slightly lower level is +validates_each+. It provides some of the same options as the purely declarative
    # validation methods, but like all the lower-level approaches it requires manually adding to the errors collection
    # when the record is invalid.
    #
    # At a yet lower level, a model can use the class methods +validate+, +validate_on_create+ and +validate_on_update+
    # to add validation methods or blocks. These are ActiveSupport::Callbacks and follow the same rules of inheritance
    # and chaining.
    #
    # The lowest level style is to define the instance methods +validate+, +validate_on_create+ and +validate_on_update+
    # as documented in ActiveRecord::Validations.
    #
    # == +validate+, +validate_on_create+ and +validate_on_update+ Class Methods
    #
    # Calls to these methods add a validation method or block to the class. Again, this approach is recommended
    # only when the higher-level methods documented below (<tt>validates_..._of</tt> and +validates_associated+) are
    # insufficient to handle the required validation.
    #
    # This can be done with a symbol pointing to a method:
    #
    #   class Comment < ActiveRecord::Base
    #     validate :must_be_friends
    #
    #     def must_be_friends
    #       errors.add_to_base("Must be friends to leave a comment") unless commenter.friend_of?(commentee)
    #     end
    #   end
    #
    # Or with a block which is passed the current record to be validated:
    #
    #   class Comment < ActiveRecord::Base
    #     validate do |comment|
    #       comment.must_be_friends
    #     end
    #
    #     def must_be_friends
    #       errors.add_to_base("Must be friends to leave a comment") unless commenter.friend_of?(commentee)
    #     end
    #   end
    #
    # This usage applies to +validate_on_create+ and +validate_on_update+ as well.
    module ClassMethods
      DEFAULT_VALIDATION_OPTIONS = {
        :on => :save,
        :allow_nil => false,
        :allow_blank => false,
        :message => nil
      }.freeze

      ALL_RANGE_OPTIONS = [ :is, :within, :in, :minimum, :maximum ].freeze
      ALL_NUMERICALITY_CHECKS = { :greater_than => '>', :greater_than_or_equal_to => '>=',
                                  :equal_to => '==', :less_than => '<', :less_than_or_equal_to => '<=',
                                  :odd => 'odd?', :even => 'even?' }.freeze

      # Validates each attribute against a block.
      #
      #   class Person < ActiveRecord::Base
      #     validates_each :first_name, :last_name do |record, attr, value|
      #       record.errors.add attr, 'starts with z.' if value[0] == ?z
      #     end
      #   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>: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
      #   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_each(*attrs)
        options = attrs.extract_options!.symbolize_keys
        attrs   = attrs.flatten

        # Declare the validation.
        send(validation_method(options[:on] || :save), options) do |record|
          attrs.each do |attr|
            value = record.send(attr)
            next if (value.nil? && options[:allow_nil]) || (value.blank? && options[:allow_blank])
            yield record, attr, value
          end
        end
      end

      # Encapsulates the pattern of wanting to validate a password or email address field with a confirmation. Example:
      #
      #   Model:
      #     class Person < ActiveRecord::Base
      #       validates_confirmation_of :user_name, :password
      #       validates_confirmation_of :email_address, :message => "should match confirmation"
      #     end
      #
      #   View:
      #     <%= password_field "person", "password" %>
      #     <%= password_field "person", "password_confirmation" %>
      #
      # The added +password_confirmation+ attribute is virtual; it exists only as an in-memory attribute for validating the password.
      # To achieve this, the validation adds accessors to the model for the confirmation attribute. NOTE: This check is performed
      # only if +password_confirmation+ is not +nil+, and by default only on save. To require confirmation, make sure to add a presence
      # check for the confirmation attribute:
      #
      #   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>: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_confirmation_of(*attr_names)
        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|
          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

      # Encapsulates the pattern of wanting to validate the acceptance of a terms of service check box (or similar agreement). Example:
      #
      #   class Person < ActiveRecord::Base
      #     validates_acceptance_of :terms_of_service
      #     validates_acceptance_of :eula, :message => "must be abided"
      #   end
      #
      # 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>: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 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.
      # * <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_acceptance_of(*attr_names)
        configuration = { :on => :save, :allow_nil => true, :accept => "1" }
        configuration.update(attr_names.extract_options!)

        db_cols = begin
          column_names
        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|
          unless value == configuration[:accept]
            record.errors.add(attr_name, :accepted, :default => configuration[:message])
          end
        end
      end

      # Validates that the specified attributes are not blank (as defined by Object#blank?). Happens by default on save. Example:
      #
      #   class Person < ActiveRecord::Base
      #     validates_presence_of :first_name
      #   end
      #
      # 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 <tt>validates_inclusion_of :field_name, :in => [true, false]</tt>.
      #
      # 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.
      #
      def validates_presence_of(*attr_names)
        configuration = { :on => :save }
        configuration.update(attr_names.extract_options!)

        # can't use validates_each here, because it cannot cope with nonexistent attributes,
        # while errors.add_on_empty can
        send(validation_method(configuration[:on]), configuration) do |record|
          record.errors.add_on_blank(attr_names, configuration[:message])
        end
      end

      # 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 %{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 %{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>: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 %{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 = {
          :tokenizer => lambda {|value| value.split(//)}
        }.merge(DEFAULT_VALIDATION_OPTIONS)
        options.update(attrs.extract_options!.symbolize_keys)

        # Ensure that one and only one range option is specified.
        range_options = ALL_RANGE_OPTIONS & options.keys
        case range_options.size
          when 0
            raise ArgumentError, 'Range unspecified.  Specify the :within, :maximum, :minimum, or :is option.'
          when 1
            # Valid number of options; do nothing.
          else
            raise ArgumentError, 'Too many range options specified.  Choose only one.'
        end

        # 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)

            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
            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 => "<=" }

            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

      alias_method :validates_size_of, :validates_length_of


      # Validates whether the value of the specified attributes are unique across the system. Useful for making sure that only one user
      # can be named "davidhh".
      #
      #   class Person < ActiveRecord::Base
      #     validates_uniqueness_of :user_name, :scope => :account_id
      #   end
      #
      # It can also validate whether the value of the specified attributes are unique based on multiple scope parameters.  For example,
      # making sure that a teacher can only be on the schedule once per semester for a particular class.
      #
      #   class TeacherSchedule < ActiveRecord::Base
      #     validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id]
      #   end
      #
      # 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.
      #
      # Configuration options:
      # * <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>: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 = { :case_sensitive => true }
        configuration.update(attr_names.extract_options!)

        validates_each(attr_names,configuration) do |record, attr_name, value|
          # The check for an existing value should be run from a class that
          # isn't abstract. This means working down from the current class
          # (self), to the first non-abstract class. Since classes don't know
          # their subclasses, we have to build the hierarchy between self and
          # the record's class.
          class_hierarchy = [record.class]
          while class_hierarchy.first != self
            class_hierarchy.insert(0, class_hierarchy.first.superclass)
          end

          # Now we can work our way down the tree to the first non-abstract
          # class (which has a database table to query from).
          finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }

          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.mb_chars[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
            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 " << attribute_condition("#{record.class.quoted_table_name}.#{connection.quote_column_name(scope_item)}", scope_value)
              condition_params << scope_value
            end
          end

          unless record.new_record?
            condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?"
            condition_params << record.send(:id)
          end

          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
          end
        end
      end


      # Validates whether the value of the specified attribute is of the correct form by matching it against the regular expression
      # provided.
      #
      #   class Person < ActiveRecord::Base
      #     validates_format_of :email, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, :on => :create
      #   end
      #
      # Note: use <tt>\A</tt> and <tt>\Z</tt> to match the start and end of the string, <tt>^</tt> and <tt>$</tt> match the start/end of a line.
      #
      # 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>: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_format_of(*attr_names)
        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|
          unless value.to_s =~ configuration[:with]
            record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value)
          end
        end
      end

      # Validates whether the value of the specified attribute is available in a particular enumerable object.
      #
      #   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 %{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 +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.
      def validates_inclusion_of(*attr_names)
        configuration = { :on => :save }
        configuration.update(attr_names.extract_options!)

        enum = configuration[:in] || configuration[:within]

        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|
          unless enum.include?(value)
            record.errors.add(attr_name, :inclusion, :default => configuration[:message], :value => value)
          end
        end
      end

      # Validates that the value of the specified attribute is not in a particular enumerable object.
      #
      #   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 %{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>: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_exclusion_of(*attr_names)
        configuration = { :on => :save }
        configuration.update(attr_names.extract_options!)

        enum = configuration[:in] || configuration[:within]

        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|
          if enum.include?(value)
            record.errors.add(attr_name, :exclusion, :default => configuration[:message], :value => value)
          end
        end
      end

      # Validates whether the associated object or objects are all valid themselves. Works with any kind of association.
      #
      #   class Book < ActiveRecord::Base
      #     has_many :pages
      #     belongs_to :library
      #
      #     validates_associated :pages, :library
      #   end
      #
      # Warning: If, after the above definition, you then wrote:
      #
      #   class Page < ActiveRecord::Base
      #     belongs_to :book
      #
      #     validates_associated :book
      #   end
      #
      # 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>: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_associated(*attr_names)
        configuration = { :on => :save }
        configuration.update(attr_names.extract_options!)

        validates_each(attr_names, configuration) do |record, attr_name, value|
          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

      # Validates whether the value of the specified attribute is numeric by trying to convert it to
      # 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
      #   end
      #
      # Configuration options:
      # * <tt>:message</tt> - A custom error message (default is: "is not a number").
      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
      # * <tt>:only_integer</tt> - Specifies whether the value has to be an integer, e.g. an integral value (default is +false+).
      # * <tt>:allow_nil</tt> - Skip validation if attribute is +nil+ (default is +false+). Notice that for fixnum and float columns empty strings are converted to +nil+.
      # * <tt>:greater_than</tt> - Specifies the value must be greater than the supplied value.
      # * <tt>:greater_than_or_equal_to</tt> - Specifies the value must be greater than or equal the supplied value.
      # * <tt>:equal_to</tt> - Specifies the value must be equal to the supplied value.
      # * <tt>:less_than</tt> - Specifies the value must be less than the supplied value.
      # * <tt>:less_than_or_equal_to</tt> - Specifies the value must be less than or equal the supplied value.
      # * <tt>:odd</tt> - Specifies the value must be an odd number.
      # * <tt>:even</tt> - Specifies the value must be an even number.
      # * <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_numericality_of(*attr_names)
        configuration = { :on => :save, :only_integer => false, :allow_nil => false }
        configuration.update(attr_names.extract_options!)


        numericality_options = ALL_NUMERICALITY_CHECKS.keys & configuration.keys

        (numericality_options - [ :odd, :even ]).each do |option|
          raise ArgumentError, ":#{option} must be a number" unless configuration[option].is_a?(Numeric)
        end

        validates_each(attr_names,configuration) do |record, attr_name, value|
          raw_value = record.send("#{attr_name}_before_type_cast") || value

          next if configuration[:allow_nil] and raw_value.nil?

          if configuration[:only_integer]
            unless raw_value.to_s =~ /\A[+-]?\d+\Z/
              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)
            rescue ArgumentError, TypeError
              record.errors.add(attr_name, :not_a_number, :value => raw_value, :default => configuration[:message])
              next
            end
          end

          numericality_options.each do |option|
            case option
              when :odd, :even
                unless raw_value.to_i.method(ALL_NUMERICALITY_CHECKS[option])[]
                  record.errors.add(attr_name, option, :value => raw_value, :default => configuration[:message])
                end
              else
                record.errors.add(attr_name, option, :default => configuration[:message], :value => raw_value, :count => configuration[option]) unless raw_value.method(ALL_NUMERICALITY_CHECKS[option])[configuration[option]]
            end
          end
        end
      end

      # Creates an object just like Base.create but calls save! instead of save
      # so an exception is raised if the record is invalid.
      def create!(attributes = nil, &block)
        if attributes.is_a?(Array)
          attributes.collect { |attr| create!(attr, &block) }
        else
          object = new(attributes)
          yield(object) if block_given?
          object.save!
          object
        end
      end

      private
        def validation_method(on)
          case on
            when :save   then :validate
            when :create then :validate_on_create
            when :update then :validate_on_update
          end
        end
    end

    # The validation process on save can be skipped by passing false. The regular Base#save method is
    # replaced with this when the validations module is mixed in, which it is by default.
    def save_with_validation(perform_validation = true)
      if perform_validation && valid? || !perform_validation
        save_without_validation
      else
        false
      end
    end

    # Attempts to save the record just like Base#save but will raise a RecordInvalid exception instead of returning false
    # if the record is not valid.
    def save_with_validation!
      if valid?
        save_without_validation!
      else
        raise RecordInvalid.new(self)
      end
    end

    # Runs +validate+ and +validate_on_create+ or +validate_on_update+ and returns true if no errors were added otherwise false.
    def valid?
      errors.clear

      run_callbacks(:validate)
      validate

      if new_record?
        run_callbacks(:validate_on_create)
        validate_on_create
      else
        run_callbacks(:validate_on_update)
        validate_on_update
      end

      errors.empty?
    end

    # Performs the opposite of <tt>valid?</tt>. Returns true if errors were added, false otherwise.
    def invalid?
      !valid?
    end

    # Returns the Errors object that holds all information about attribute error messages.
    def errors
      @errors ||= Errors.new(self)
    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
