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.

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.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 795
795:     def errors
796:       @errors ||= Errors.new(self)
797:     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.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 750
750:     def save_with_validation(perform_validation = true)
751:       if perform_validation && valid? || !perform_validation
752:         save_without_validation
753:       else
754:         false
755:       end
756:     end
save_with_validation!()

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.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 760
760:     def save_with_validation!
761:       if valid?
762:         save_without_validation!
763:       else
764:         raise RecordInvalid.new(self)
765:       end
766:     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.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 771
771:     def update_attribute_with_validation_skipping(name, value)
772:       send(name.to_s + '=', value)
773:       save(false)
774:     end
valid?()

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

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 777
777:     def valid?
778:       errors.clear
779: 
780:       run_validations(:validate)
781:       validate
782: 
783:       if new_record?
784:         run_validations(:validate_on_create)
785:         validate_on_create
786:       else
787:         run_validations(:validate_on_update)
788:         validate_on_update
789:       end
790: 
791:       errors.empty?
792:     end
Protected Instance methods
validate(

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

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 801
801:       def validate #:doc:
802:       end
validate_on_create(

Overwrite this method for validation checks used only on creation.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 805
805:       def validate_on_create #:doc:
806:       end
validate_on_update(

Overwrite this method for validation checks used only on updates.

     # File vendor/rails/activerecord/lib/active_record/validations.rb, line 809
809:       def validate_on_update # :doc:
810:       end