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.count                        # => 2
  person.errors.on "last_name"        # => "can't be empty"
  person.errors.on "phone_number"     # => "has invalid format"
  person.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.

Please do have a look at ActiveRecord::Validations::ClassMethods for a higher level of validations.

Methods
Classes and Modules
Module ActiveRecord::Validations::ClassMethods
Constants
VALIDATIONS = %w( validate validate_on_create validate_on_update )
Public Instance methods
errors()

Returns the Errors object that holds all information about attribute error messages.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 591
591:     def errors
592:       @errors = Errors.new(self) if @errors.nil?
593:       @errors
594:     end
save!()

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.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 560
560:     def save!
561:       valid? ? save_without_validation : raise(RecordInvalid)
562:     end
save_with_validation(perform_validation = true)

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.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 554
554:     def save_with_validation(perform_validation = true)
555:       if perform_validation && valid? || !perform_validation then save_without_validation else false end
556:     end
update_attribute_with_validation_skipping(name, value)

Updates a single attribute and saves the record without going through the normal validation procedure. This is especially useful for boolean flags on existing records. The regular update_attribute method in Base is replaced with this when the validations module is mixed in, which it is by default.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 567
567:     def update_attribute_with_validation_skipping(name, value)
568:       self[name] = value
569:       save(false)
570:     end
valid?()

Runs validate and validate_on_create or validate_on_update and returns true if no errors were added otherwise false.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 573
573:     def valid?
574:       errors.clear
575: 
576:       run_validations(:validate)
577:       validate
578: 
579:       if new_record?
580:         run_validations(:validate_on_create)
581:         validate_on_create 
582:       else
583:         run_validations(:validate_on_update)
584:         validate_on_update
585:       end
586: 
587:       errors.empty?
588:     end
Protected Instance methods
validate(

Overwrite this method for validation checks on all saves and use Errors.add(field, msg) for invalid attributes.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 598
598:       def validate #:doc:
599:       end
validate_on_create(

Overwrite this method for validation checks used only on creation.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 602
602:       def validate_on_create #:doc:
603:       end
validate_on_update(

Overwrite this method for validation checks used only on updates.

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 606
606:       def validate_on_update # :doc:
607:       end