Active Record classes can implement validations in several ways. The highest level, easiest to read, and recommended approach is to use the declarative validates_..._ofclass 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_createand validate_on_updateto 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_createand validate_on_updateas documented in ActiveRecord::Validations.

validate, validate_on_createand 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_..._ofand 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_createand validate_on_updateas well.

Methods
C
V
Constants
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
 
Instance Public methods
create!(attributes = nil, &block)

Creates an object just like ActiveRecord::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 1064
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
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_serviceis not niland 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 trueif you are validating a database column, since the attribute is typecast from “1” to truebefore 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 548
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_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 981
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_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_confirmationattribute 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_confirmationis 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 512
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
validates_each(*attrs)

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 470
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
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 935
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_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/, :on => :create
end

Note: use \Aand \Zto 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 869
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_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 901
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_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 :zip_code, :minimum => 5, :too_short => "please enter at least %{count} characters"
  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 :ismethod 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.

Also aliased as: validates_size_of
# File activerecord/lib/active_record/validations.rb, line 637
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
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_integeris false) or applying it to the regular expression /\A[+\-]?\d+\Z/(if only_integeris 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 1018
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
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 591
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_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 ( trueby 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 784
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