Active Record classes can implement validations in several ways. The highest level, easiest to read, and recommended approach is to use the declarative validates_..._of 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 (validates_..._of 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.

Methods
Constants
DEFAULT_VALIDATION_OPTIONS = { :on => :save, :allow_nil => false, :allow_blank => false, :message => nil
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
Public Instance methods
create!(attributes = nil) {|object| ...}

Creates an object just like Base.create but calls save! instead of save so an exception is raised if the record is invalid.

      # File activerecord/lib/active_record/validations.rb, line 1065
1065:       def create!(attributes = nil, &block)
1066:         if attributes.is_a?(Array)
1067:           attributes.collect { |attr| create!(attr, &block) }
1068:         else
1069:           object = new(attributes)
1070:           yield(object) if block_given?
1071:           object.save!
1072:           object
1073:         end
1074:       end
validates_acceptance_of(*attr_names)

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:

  • :message - A custom error message (default is: "must be accepted").
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :allow_nil - Skip validation if attribute is nil (default is true).
  • :accept - 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.
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 549
549:       def validates_acceptance_of(*attr_names)
550:         configuration = { :on => :save, :allow_nil => true, :accept => "1" }
551:         configuration.update(attr_names.extract_options!)
552: 
553:         db_cols = begin
554:           column_names
555:         rescue Exception # To ignore both statement and connection errors
556:           []
557:         end
558:         names = attr_names.reject { |name| db_cols.include?(name.to_s) }
559:         attr_accessor(*names)
560: 
561:         validates_each(attr_names,configuration) do |record, attr_name, value|
562:           unless value == configuration[:accept]
563:             record.errors.add(attr_name, :accepted, :default => configuration[:message])
564:           end
565:         end
566:       end
validates_associated(*attr_names)

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:

  • :message - A custom error message (default is: "is invalid")
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 982
982:       def validates_associated(*attr_names)
983:         configuration = { :on => :save }
984:         configuration.update(attr_names.extract_options!)
985: 
986:         validates_each(attr_names, configuration) do |record, attr_name, value|
987:           unless (value.is_a?(Array) ? value : [value]).collect { |r| r.nil? || r.valid? }.all?
988:             record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value)
989:           end
990:         end
991:       end
validates_confirmation_of(*attr_names)

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:

  • :message - A custom error message (default is: "doesn‘t match confirmation").
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 513
513:       def validates_confirmation_of(*attr_names)
514:         configuration = { :on => :save }
515:         configuration.update(attr_names.extract_options!)
516: 
517:         attr_accessor(*(attr_names.map { |n| "#{n}_confirmation" }))
518: 
519:         validates_each(attr_names, configuration) do |record, attr_name, value|
520:           unless record.send("#{attr_name}_confirmation").nil? or value == record.send("#{attr_name}_confirmation")
521:             record.errors.add(attr_name, :confirmation, :default => configuration[:message])
522:           end
523:         end
524:       end
validates_each(*attrs) {|record, attr, value| ...}

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:

  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :allow_nil - Skip validation if attribute is nil.
  • :allow_blank - Skip validation if attribute is blank.
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 471
471:       def validates_each(*attrs)
472:         options = attrs.extract_options!.symbolize_keys
473:         attrs   = attrs.flatten
474: 
475:         # Declare the validation.
476:         send(validation_method(options[:on] || :save), options) do |record|
477:           attrs.each do |attr|
478:             value = record.send(attr)
479:             next if (value.nil? && options[:allow_nil]) || (value.blank? && options[:allow_blank])
480:             yield record, attr, value
481:           end
482:         end
483:       end
validates_exclusion_of(*attr_names)

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:

  • :in - An enumerable object of items that the value shouldn‘t be part of.
  • :message - Specifies a custom error message (default is: "is reserved").
  • :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).
  • :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 936
936:       def validates_exclusion_of(*attr_names)
937:         configuration = { :on => :save }
938:         configuration.update(attr_names.extract_options!)
939: 
940:         enum = configuration[:in] || configuration[:within]
941: 
942:         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?)
943: 
944:         validates_each(attr_names, configuration) do |record, attr_name, value|
945:           if enum.include?(value)
946:             record.errors.add(attr_name, :exclusion, :default => configuration[:message], :value => value)
947:           end
948:         end
949:       end
validates_format_of(*attr_names)

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 \A and \Z to match the start and end of the string, ^ and $ match the start/end of a line.

A regular expression must be provided or else an exception will be raised.

Configuration options:

  • :message - A custom error message (default is: "is invalid").
  • :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).
  • :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).
  • :with - The regular expression used to validate the format with (note: must be supplied!).
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 870
870:       def validates_format_of(*attr_names)
871:         configuration = { :on => :save, :with => nil }
872:         configuration.update(attr_names.extract_options!)
873: 
874:         raise(ArgumentError, "A regular expression must be supplied as the :with option of the configuration hash") unless configuration[:with].is_a?(Regexp)
875: 
876:         validates_each(attr_names, configuration) do |record, attr_name, value|
877:           unless value.to_s =~ configuration[:with]
878:             record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value)
879:           end
880:         end
881:       end
validates_inclusion_of(*attr_names)

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:

  • :in - An enumerable object of available items.
  • :message - Specifies a custom error message (default is: "is not included in the list").
  • :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).
  • :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 902
902:       def validates_inclusion_of(*attr_names)
903:         configuration = { :on => :save }
904:         configuration.update(attr_names.extract_options!)
905: 
906:         enum = configuration[:in] || configuration[:within]
907: 
908:         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?)
909: 
910:         validates_each(attr_names, configuration) do |record, attr_name, value|
911:           unless enum.include?(value)
912:             record.errors.add(attr_name, :inclusion, :default => configuration[:message], :value => value)
913:           end
914:         end
915:       end
validates_length_of(*attrs)

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:

  • :minimum - The minimum size of the attribute.
  • :maximum - The maximum size of the attribute.
  • :is - The exact size of the attribute.
  • :within - A range specifying the minimum and maximum size of the attribute.
  • :in - A synonym(or alias) for :within.
  • :allow_nil - Attribute may be nil; skip validation.
  • :allow_blank - Attribute may be blank; skip validation.
  • :too_long - The error message if the attribute goes over the maximum (default is: "is too long (maximum is %{count} characters)").
  • :too_short - The error message if the attribute goes under the minimum (default is: "is too short (min is %{count} characters)").
  • :wrong_length - The error message if using the :is method and the attribute is the wrong size (default is: "is the wrong length (should be %{count} characters)").
  • :message - The error message to use for a :minimum, :maximum, or :is violation. An alias of the appropriate too_long/too_short/wrong_length message.
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :tokenizer - Specifies how to split up the attribute string. (e.g. :tokenizer => lambda {|str| str.scan(/\w+/)} to count words as in above example.) Defaults to lambda{ |value| value.split(//) } which counts individual characters.
This method is also aliased as validates_size_of
     # File activerecord/lib/active_record/validations.rb, line 638
638:       def validates_length_of(*attrs)
639:         # Merge given options with defaults.
640:         options = {
641:           :tokenizer => lambda {|value| value.split(//)}
642:         }.merge(DEFAULT_VALIDATION_OPTIONS)
643:         options.update(attrs.extract_options!.symbolize_keys)
644: 
645:         # Ensure that one and only one range option is specified.
646:         range_options = ALL_RANGE_OPTIONS & options.keys
647:         case range_options.size
648:           when 0
649:             raise ArgumentError, 'Range unspecified.  Specify the :within, :maximum, :minimum, or :is option.'
650:           when 1
651:             # Valid number of options; do nothing.
652:           else
653:             raise ArgumentError, 'Too many range options specified.  Choose only one.'
654:         end
655: 
656:         # Get range option and value.
657:         option = range_options.first
658:         option_value = options[range_options.first]
659:         key = {:is => :wrong_length, :minimum => :too_short, :maximum => :too_long}[option]
660:         custom_message = options[:message] || options[key]
661: 
662:         case option
663:           when :within, :in
664:             raise ArgumentError, ":#{option} must be a Range" unless option_value.is_a?(Range)
665: 
666:             validates_each(attrs, options) do |record, attr, value|
667:               value = options[:tokenizer].call(value) if value.kind_of?(String)
668:               if value.nil? or value.size < option_value.begin
669:                 record.errors.add(attr, :too_short, :default => custom_message || options[:too_short], :count => option_value.begin)
670:               elsif value.size > option_value.end
671:                 record.errors.add(attr, :too_long, :default => custom_message || options[:too_long], :count => option_value.end)
672:               end
673:             end
674:           when :is, :minimum, :maximum
675:             raise ArgumentError, ":#{option} must be a nonnegative Integer" unless option_value.is_a?(Integer) and option_value >= 0
676: 
677:             # Declare different validations per option.
678:             validity_checks = { :is => "==", :minimum => ">=", :maximum => "<=" }
679: 
680:             validates_each(attrs, options) do |record, attr, value|
681:               value = options[:tokenizer].call(value) if value.kind_of?(String)
682:               unless !value.nil? and value.size.method(validity_checks[option])[option_value]
683:                 record.errors.add(attr, key, :default => custom_message, :count => option_value)
684:               end
685:             end
686:         end
687:       end
validates_numericality_of(*attr_names)

Validates whether the value of the specified attribute is numeric by trying to convert it to a float with Kernel.Float (if only_integer is false) or applying it to the regular expression /\A[+\-]?\d+\Z/ (if only_integer is set to true).

  class Person < ActiveRecord::Base
    validates_numericality_of :value, :on => :create
  end

Configuration options:

  • :message - A custom error message (default is: "is not a number").
  • :on - Specifies when this validation is active (default is :save, other options :create, :update).
  • :only_integer - Specifies whether the value has to be an integer, e.g. an integral value (default is false).
  • :allow_nil - Skip validation if attribute is nil (default is false). Notice that for fixnum and float columns empty strings are converted to nil.
  • :greater_than - Specifies the value must be greater than the supplied value.
  • :greater_than_or_equal_to - Specifies the value must be greater than or equal the supplied value.
  • :equal_to - Specifies the value must be equal to the supplied value.
  • :less_than - Specifies the value must be less than the supplied value.
  • :less_than_or_equal_to - Specifies the value must be less than or equal the supplied value.
  • :odd - Specifies the value must be an odd number.
  • :even - Specifies the value must be an even number.
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
      # File activerecord/lib/active_record/validations.rb, line 1019
1019:       def validates_numericality_of(*attr_names)
1020:         configuration = { :on => :save, :only_integer => false, :allow_nil => false }
1021:         configuration.update(attr_names.extract_options!)
1022: 
1023: 
1024:         numericality_options = ALL_NUMERICALITY_CHECKS.keys & configuration.keys
1025: 
1026:         (numericality_options - [ :odd, :even ]).each do |option|
1027:           raise ArgumentError, ":#{option} must be a number" unless configuration[option].is_a?(Numeric)
1028:         end
1029: 
1030:         validates_each(attr_names,configuration) do |record, attr_name, value|
1031:           raw_value = record.send("#{attr_name}_before_type_cast") || value
1032: 
1033:           next if configuration[:allow_nil] and raw_value.nil?
1034: 
1035:           if configuration[:only_integer]
1036:             unless raw_value.to_s =~ /\A[+-]?\d+\Z/
1037:               record.errors.add(attr_name, :not_a_number, :value => raw_value, :default => configuration[:message])
1038:               next
1039:             end
1040:             raw_value = raw_value.to_i
1041:           else
1042:             begin
1043:               raw_value = Kernel.Float(raw_value)
1044:             rescue ArgumentError, TypeError
1045:               record.errors.add(attr_name, :not_a_number, :value => raw_value, :default => configuration[:message])
1046:               next
1047:             end
1048:           end
1049: 
1050:           numericality_options.each do |option|
1051:             case option
1052:               when :odd, :even
1053:                 unless raw_value.to_i.method(ALL_NUMERICALITY_CHECKS[option])[]
1054:                   record.errors.add(attr_name, option, :value => raw_value, :default => configuration[:message])
1055:                 end
1056:               else
1057:                 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]]
1058:             end
1059:           end
1060:         end
1061:       end
validates_presence_of(*attr_names)

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 validates_inclusion_of :field_name, :in => [true, false].

This is due to the way Object#blank? handles boolean values: false.blank? # => true.

Configuration options:

  • message - A custom error message (default is: "can‘t be blank").
  • on - Specifies when this validation is active (default is :save, other options :create, :update).
  • if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.
     # File activerecord/lib/active_record/validations.rb, line 592
592:       def validates_presence_of(*attr_names)
593:         configuration = { :on => :save }
594:         configuration.update(attr_names.extract_options!)
595: 
596:         # can't use validates_each here, because it cannot cope with nonexistent attributes,
597:         # while errors.add_on_empty can
598:         send(validation_method(configuration[:on]), configuration) do |record|
599:           record.errors.add_on_blank(attr_names, configuration[:message])
600:         end
601:       end
validates_size_of(*attrs)
validates_uniqueness_of(*attr_names)

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:

  • :message - Specifies a custom error message (default is: "has already been taken").
  • :scope - One or more columns by which to limit the scope of the uniqueness constraint.
  • :case_sensitive - Looks for an exact match. Ignored by non-text columns (true by default).
  • :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).
  • :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).
  • :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
  • :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). 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: 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.

     # File activerecord/lib/active_record/validations.rb, line 785
785:       def validates_uniqueness_of(*attr_names)
786:         configuration = { :case_sensitive => true }
787:         configuration.update(attr_names.extract_options!)
788: 
789:         validates_each(attr_names,configuration) do |record, attr_name, value|
790:           # The check for an existing value should be run from a class that
791:           # isn't abstract. This means working down from the current class
792:           # (self), to the first non-abstract class. Since classes don't know
793:           # their subclasses, we have to build the hierarchy between self and
794:           # the record's class.
795:           class_hierarchy = [record.class]
796:           while class_hierarchy.first != self
797:             class_hierarchy.insert(0, class_hierarchy.first.superclass)
798:           end
799: 
800:           # Now we can work our way down the tree to the first non-abstract
801:           # class (which has a database table to query from).
802:           finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }
803: 
804:           column = finder_class.columns_hash[attr_name.to_s]
805: 
806:           if value.nil?
807:             comparison_operator = "IS ?"
808:           elsif column.text?
809:             comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
810:             value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s
811:           else
812:             comparison_operator = "= ?"
813:           end
814: 
815:           sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}"
816: 
817:           if value.nil? || (configuration[:case_sensitive] || !column.text?)
818:             condition_sql = "#{sql_attribute} #{comparison_operator}"
819:             condition_params = [value]
820:           else
821:             condition_sql = "LOWER(#{sql_attribute}) #{comparison_operator}"
822:             condition_params = [value.mb_chars.downcase]
823:           end
824: 
825:           if scope = configuration[:scope]
826:             Array(scope).map do |scope_item|
827:               scope_value = record.send(scope_item)
828:               condition_sql << " AND " << attribute_condition("#{record.class.quoted_table_name}.#{connection.quote_column_name(scope_item)}", scope_value)
829:               condition_params << scope_value
830:             end
831:           end
832: 
833:           unless record.new_record?
834:             condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?"
835:             condition_params << record.send(:id)
836:           end
837: 
838:           finder_class.with_exclusive_scope do
839:             if finder_class.exists?([condition_sql, *condition_params])
840:               record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value)
841:             end
842:           end
843:         end
844:       end