Active Record Validations and Callbacks

This guide teaches you how to work with the lifecycle of your Active Record objects. More precisely, you will learn how to validate the state of your objects before they go into the database and also how to teach them to perform custom operations at certain points of their lifecycles.

After reading this guide and trying out the presented concepts, we hope that you'll be able to:

Correctly use all the built-in Active Record validation helpers
Create your own custom validation methods
Work with the error messages generated by the validation proccess
Register callback methods that will execute custom operations during your objects lifecycle, for example before/after they are saved.
Create special classes that encapsulate common behaviour for your callbacks
Create Observers - classes with callback methods specific for each of your models, keeping the callback code outside your models' declarations.

1. Motivations to validate your Active Record objects

The main reason for validating your objects before they get into the database is to ensure that only valid data is recorded. It's important to be sure that an email address column only contains valid email addresses, or that the customer's name column will never be empty. Constraints like that keep your database organized and helps your application to work properly.

There are several ways to validate the data that goes to the database, like using database native constraints, implementing validations only at the client side or implementing them directly into your models. Each one has pros and cons:

Using database constraints and/or stored procedures makes the validation mechanisms database-dependent and may turn your application into a hard to test and mantain beast. However, if your database is used by other applications, it may be a good idea to use some constraints also at the database level.
Implementing validations only at the client side can be problematic, specially with web-based applications. Usually this kind of validation is done using javascript, which may be turned off in the user's browser, leading to invalid data getting inside your database. However, if combined with server side validation, client side validation may be useful, since the user can have a faster feedback from the application when trying to save invalid data.
Using validation directly into your Active Record classes ensures that only valid data gets recorded, while still keeping the validation code in the right place, avoiding breaking the MVC pattern. Since the validation happens on the server side, the user cannot disable it, so it's also safer. It may be a hard and tedious work to implement some of the logic involved in your models' validations, but fear not: Active Record gives you the hability to easily create validations, using several built-in helpers while still allowing you to create your own validation methods.

2. How it works

2.1. When does validation happens?

There are two kinds of Active Record objects: those that correspond to a row inside your database and those who do not. When you create a fresh object, using the new method, that object does not belong to the database yet. Once you call save upon that object it'll be recorded to it's table. Active Record uses the new_record? instance method to discover if an object is already in the database or not. Consider the following simple and very creative Active Record class:

class Person < ActiveRecord::Base
end

We can see how it works by looking at the following script/console output:

>> p = Person.new(:name => "John Doe", :birthdate => Date.parse("09/03/1979"))
=> #<Person id: nil, name: "John Doe", birthdate: "1979-09-03", created_at: nil, updated_at: nil>
>> p.new_record?
=> true
>> p.save
=> true
>> p.new_record?
=> false

Saving new records means sending an SQL insert operation to the database, while saving existing records (by calling either save or update_attributes) will result in a SQL update operation. Active Record will use this facts to perform validations upon your objects, avoiding then to be recorded to the database if their inner state is invalid in some way. You can specify validations that will be beformed every time a object is saved, just when you're creating a new record or when you're updating an existing one.

There are four methods that when called will trigger validation: save, save!, update_attributes and update_attributes!. There is one method left, which is update_attribute. This method will update the value of an attribute without triggering any validation, so be careful when using update_attribute, since it can let you save your objects in an invalid state.

2.2. The meaning of valid

For verifying if an object is valid, Active Record uses the valid? method, which basically looks inside the object to see if it has any validation errors. These errors live in a collection that can be accessed through the errors instance method. The proccess is really simple: If the errors method returns an empty collection, the object is valid and can be saved. Each time a validation fails, an error message is added to the errors collection.

3. The declarative validation helpers

Active Record offers many pre-defined validation helpers that you can use directly inside your class definitions. These helpers create validations rules that are commonly used in most of the applications that you'll write, so you don't need to recreate it everytime, avoiding code duplication, keeping everything organized and boosting your productivity. Everytime a validation fails, an error message is added to the object's errors collection, this message being associated with the field being validated.

Each helper accepts an arbitrary number of attributes, received as symbols, so with a single line of code you can add the same kind of validation to several attributes.

All these helpers accept the :on and :message options, which define when the validation should be applied and what message should be added to the errors collection when it fails, respectively. The :on option takes one the values :save (it's the default), :create or :update. There is a default error message for each one of the validation helpers. These messages are used when the :message option isn't used. Let's take a look at each one of the available helpers, listed in alphabetic order.

3.1. The `validates_acceptance_of` helper

Validates that a checkbox has been checked for agreement purposes. It's normally used when the user needs to agree with your application's terms of service, confirm reading some clauses or any similar concept. This validation is very specific to web applications and actually this acceptance does not need to be recorded anywhere in your database (if you don't have a field for it, the helper will just create a virtual attribute).

class Person < ActiveRecord::Base
  validates_acceptance_of :terms_of_service
end

The default error message for validates_acceptance_of is "must be accepted"

validates_acceptance_of can receive an :accept option, which determines the value that will be considered acceptance. It defaults to "1", but you can change it.

class Person < ActiveRecord::Base
  validates_acceptance_of :terms_of_service, :accept => 'yes'
end

3.2. The `validates_associated` helper

You should use this helper when your model has associations with other models and they also need to be validated. When you try to save your object, valid? will be called upon each one of the associated objects.

class Library < ActiveRecord::Base
  has_many :books
  validates_associated :books
end

This validation will work with all the association types.

Pay attention not to use validates_associated on both ends of your associations, because this will lead to several recursive calls and blow up the method calls' stack.

The default error message for validates_associated is "is invalid". Note that the errors for each failed validation in the associated objects will be set there and not in this model.

3.3. The `validates_confirmation_of` helper

You should use this helper when you have two text fields that should receive exactly the same content, like when you want to confirm an email address or password. This validation creates a virtual attribute, using the name of the field that has to be confirmed with _confirmation appended.

class Person < ActiveRecord::Base
  validates_confirmation_of :email
end

In your view template you could use something like

<%= text_field :person, :email %>
<%= text_field :person, :email_confirmation %>

This check is performed only if email_confirmation is not nil, and by default only on save. To require confirmation, make sure to add a presence check for the confirmation attribute (we'll take a look at validates_presence_of later on this guide):

class Person < ActiveRecord::Base
  validates_confirmation_of :email
  validates_presence_of :email_confirmation
end

The default error message for validates_confirmation_of is "doesn't match confirmation"

3.4. The `validates_each` helper

This helper validates attributes against a block. It doesn't have a predefined validation function. You should create one using a block, and every attribute passed to validates_each will be tested against it. In the following example, we don't want names and surnames to begin with lower case.

class Person < ActiveRecord::Base
  validates_each :name, :surname do |model, attr, value|
    model.errors.add(attr, 'Must start with upper case') if value =~ /^[a-z]/
  end
end

The block receives the model, the attribute's name and the attribute's value. If your validation fails, you can add an error message to the model, therefore making it invalid.

3.5. The `validates_exclusion_of` helper

This helper validates that the attributes' values are not included in a given set. In fact, this set can be any enumerable object.

class MovieFile < ActiveRecord::Base
  validates_exclusion_of :format, :in => %w(mov avi), :message => "Extension %s is not allowed"
end

The validates_exclusion_of helper has an option :in that receives the set of values that will not be accepted for the validated attributes. The :in option has an alias called :within that you can use for the same purpose, if you'd like to. In the previous example we used the :message option to show how we can personalize it with the current attribute's value, through the %s format mask.

The default error message for validates_exclusion_of is "is not included in the list".

3.6. The `validates_format_of` helper

This helper validates the attributes's values by testing if they match a given pattern. This pattern must be specified using a Ruby regular expression, which must be passed through the :with option.

class Product < ActiveRecord::Base
  validates_format_of :description, :with => /^[a-zA-Z]+$/, :message => "Only letters allowed"
end

The default error message for validates_format_of is "is invalid".

3.7. The `validates_inclusion_of` helper

This helper validates that the attributes' values are included in a given set. In fact, this set can be any enumerable object.

class Coffee < ActiveRecord::Base
  validates_inclusion_of :size, :in => %w(small medium large), :message => "%s is not a valid size"
end

The validates_inclusion_of helper has an option :in that receives the set of values that will be accepted. The :in option has an alias called :within that you can use for the same purpose, if you'd like to. In the previous example we used the :message option to show how we can personalize it with the current attribute's value, through the %s format mask.

The default error message for validates_inclusion_of is "is not included in the list".

3.8. The `validates_length_of` helper

This helper validates the length of your attribute's value. It can receive a variety of different options, so you can specify length contraints in different ways.

class Person < ActiveRecord::Base
  validates_length_of :name, :minimum => 2
  validates_length_of :bio, :maximum => 500
  validates_length_of :password, :in => 6..20
  validates_length_of :registration_number, :is => 6
end

The possible length constraint options are:

:minimum - The attribute cannot have less than the specified length.
:maximum - The attribute cannot have more than the specified length.
:in (or :within) - The attribute length must be included in a given interval. The value for this option must be a Ruby range.
:is - The attribute length must be equal to a given value.

The default error messages depend on the type of length validation being performed. You can personalize these messages, using the :wrong_length, :too_long and :too_short options and the %d format mask as a placeholder for the number corresponding to the length contraint being used. You can still use the :message option to specify an error message.

class Person < ActiveRecord::Base
  validates_length_of :bio, :too_long => "you're writing too much. %d characters is the maximum allowed."
end

This helper has an alias called validates_size_of, it's the same helper with a different name. You can use it if you'd like to.

3.9. The `validates_numericallity_of` helper

This helper validates that your attributes have only numeric values. By default, it will match an optional sign followed by a integral or floating point number. Using the :integer_only option set to true, you can specify that only integral numbers are allowed.

If you use :integer_only set to true, then it will use the /\A[+\-]?\d+\Z/ regular expression to validate the attribute's value. Otherwise, it will try to convert the value using Kernel.Float.

class Player < ActiveRecord::Base
  validates_numericallity_of :points
  validates_numericallity_of :games_played, :integer_only => true
end

The default error message for validates_numericallity_of is "is not a number".

3.10. The `validates_presence_of` helper

This helper validates that the attributes are not empty. It uses the blank? method to check if the value is either nil or an empty string (if the string has only spaces, it will still be considered empty).

class Person < ActiveRecord::Base
  validates_presence_of :name, :login, :email
end

If you want to be sure that an association is present, you'll need to test if the foreign key used to map the association is present, and not the associated object itself.

class LineItem < ActiveRecord::Base
  belongs_to :order
  validates_presence_of :order_id
end

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

The default error message for validates_presence_of is "can't be empty".

3.11. The `validates_uniqueness_of` helper

This helper validates that the attribute's value is unique right before the object gets saved. It does not create a uniqueness constraint directly into your database, so it may happen that two different database connections create two records with the same value for a column that you wish were unique. To avoid that, you must create an unique index in your database.

class Account < ActiveRecord::Base
  validates_uniqueness_of :email
end

The validation happens by performing a SQL query into the model's table, searching for a record where the attribute that must be validated is equal to the value in the object being validated.

There is a :scope option that you can use to specify other attributes that must be used to define uniqueness:

class Holiday < ActiveRecord::Base
  validates_uniqueness_of :name, :scope => :year, :message => "Should happen once per year"
end

There is also a :case_sensitive option that you can use to define if the uniqueness contraint will be case sensitive or not. This option defaults to true.

class Person < ActiveRecord::Base
  validates_uniqueness_of :name, :case_sensitive => false
end

The default error message for validates_uniqueness_of is "has already been taken".

4. Common validation options

There are some common options that all the validation helpers can use. Here they are, except for the :if and :unless options, which we'll cover right at the next topic.

4.1. The `:allow_nil` option

You may use the :allow_nil option everytime you just want to trigger a validation if the value being validated is not nil. You may be asking yourself if it makes any sense to use :allow_nil and validates_presence_of together. Well, it does. Remember, validation will be skipped only for nil attributes, but empty strings are not considered nil.

class Coffee < ActiveRecord::Base
  validates_inclusion_of :size, :in => %w(small medium large),
    :message => "%s is not a valid size", :allow_nil => true
end

4.2. The `:message` option

As stated before, the :message option lets you specify the message that will be added to the errors collection when validation fails. When this option is not used, Active Record will use the respective default error message for each validation helper.

4.3. The `:on` option

As stated before, the :on option lets you specify when the validation should happen. The default behaviour for all the built-in validation helpers is to be ran on save (both when you're creating a new record and when you're updating it). If you want to change it, you can use :on => :create to run the validation only when a new record is created or :on => :update to run the validation only when a record is updated.

class Person < ActiveRecord::Base
  validates_uniqueness_of :email, :on => :create # => it will be possible to update email with a duplicated value
  validates_numericallity_of :age, :on => :update # => it will be possible to create the record with a 'non-numerical age'
  validates_presence_of :name, :on => :save # => that's the default
end

5. Conditional validation

Sometimes it will make sense to validate an object just when a given predicate is satisfied. You can do that by using the :if and :unless options, which can take a symbol, a string or a Ruby Proc. You may use the :if option when you want to specify when the validation should happen. If you want to specify when the validation should not happen, then you may use the :unless option.

5.1. Using a symbol with the `:if` and `:unless` options

You can associated the :if and :unless options with a symbol corresponding to the name of a method that will get called right before validation happens. This is the most commonly used option.

class Order < ActiveRecord::Base
  validates_presence_of :card_number, :if => :paid_with_card?

  def paid_with_card?
    payment_type == "card"
  end
end

5.2. Using a string with the `:if` and `:unless` options

You can also use a string that will be evaluated using :eval and needs to contain valid Ruby code. You should use this option only when the string represents a really short condition.

class Person < ActiveRecord::Base
  validates_presence_of :surname, :if => "name.nil?"
end

5.3. Using a Proc object with the `:if` and :`unless` options

Finally, it's possible to associate :if and :unless with a Ruby Proc object which will be called. Using a Proc object can give you the hability to write a condition that will be executed only when the validation happens and not when your code is loaded by the Ruby interpreter. This option is best suited when writing short validation methods, usually one-liners.

class Account < ActiveRecord::Base
  validates_confirmation_of :password, :unless => Proc.new { |a| a.password.blank? }
end

6. Writing your own validation methods

When the built-in validation helpers are not enough for your needs, you can write your own validation methods, by implementing one or more of the validate, validate_on_create or validate_on_update methods. As the names of the methods states, the right method to implement depends on when you want the validations to be ran. The meaning of valid is still the same: to make an object invalid you just need to add a message to it's errors collection.

class Invoice < ActiveRecord::Base
  def validate_on_create
    errors.add(:expiration_date, "can't be in the past") if !expiration_date.blank? and expiration_date < Date.today
  end
end

If your validation rules are too complicated and you want to break them in small methods, you can implement all of them and call one of validate, validate_on_create or validate_on_update methods, passing it the symbols for the methods' names.

class Invoice < ActiveRecord::Base
  validate :expiration_date_cannot_be_in_the_past, :discount_cannot_be_more_than_total_value

  def expiration_date_cannot_be_in_the_past
    errors.add(:expiration_date, "can't be in the past") if !expiration_date.blank? and expiration_date < Date.today
  end

  def discount_cannot_be_greater_than_total_value
    errors.add(:discount, "can't be greater than total value") unless discount <= total_value
  end
end

7. Using the `errors` collection

You can do more than just call valid? upon your objects based on the existance of the errors collection. Here is a list of the other available methods that you can use to manipulate errors or ask for an object's state.

add_to_base lets you add errors messages that are related to the object's state as a whole, instead of being related to a specific attribute. You can use this method when you want to say that the object is invalid, no matter the values of it's attributes. add_to_base receives a string with the message.

class Person < ActiveRecord::Base
  def a_method_used_for_validation_purposes
    errors.add_to_base("This person is invalid because ...")
  end
end

add lets you manually add messages that are related to particular attributes. When writing those messages, keep in mind that Rails will prepend them with the name of the attribute that holds the error, so write it in a way that makes sense. add receives a symbol with the name of the attribute that you want to add the message to and the message itself.

class Person < ActiveRecord::Base
  def a_method_used_for_validation_purposes
    errors.add(:name, "can't have the characters !@#$%*()_-+=")
  end
end

invalid? is used when you want to check if a particular attribute is invalid. It receives a symbol with the name of the attribute that you want to check.

class Person < ActiveRecord::Base
  validates_presence_of :name, :email
end

person = Person.new(:name => "John Doe")
person.invalid?(:email) # => true

on is used when you want to check the error messages for a specific attribute. It will return different kinds of objects depending on the state of the errors collection for the given attribute. If there are no errors related to the attribute, on will return nil. If there is just one errors message for this attribute, on will return a string with the message. When errors holds two or more error messages for the attribute, on will return an array of strings, each one with one error message.

class Person < ActiveRecord::Base
  validates_presence_of :name
  validates_length_of :name, :minimum => 3
end

person = Person.new(:name => "John Doe")
person.valid? # => true
person.errors.on(:name) # => nil

person = Person.new(:name => "JD")
person.valid? # => false
person.errors.on(:name) # => "is too short (minimum is 3 characters)"

person = Person.new
person.valid? # => false
person.errors.on(:name) # => ["can't be blank", "is too short (minimum is 3 characters)"]

clear is used when you intentionally wants to clear all the messages in the errors collection.

class Person < ActiveRecord::Base
  validates_presence_of :name
  validates_length_of :name, :minimum => 3
end

person = Person.new
puts person.valid? # => false
person.errors.on(:name) # => ["can't be blank", "is too short (minimum is 3 characters)"]
person.errors.clear
person.errors # => nil

8. Callbacks

Callbacks are methods that get called at certain moments of an object's lifecycle. With callbacks it's possible to write code that will run whenever an Active Record object is created, saved, updated, deleted or loaded from the database.

8.1. Callbacks registration

In order to use the available callbacks, you need to registrate them. There are two ways of doing that.

8.2. Registering callbacks by overriding the callback methods

You can specify the callback method direcly, by overriding it. Let's see how it works using the before_validation callback, which will surprisingly run right before any validation is done.

class User < ActiveRecord::Base
  validates_presence_of :login, :email

  protected
  def before_validation
    if self.login.nil?
      self.login = email unless email.blank?
    end
  end
end

8.3. Registering callbacks by using macro-style class methods

The other way you can register a callback method is by implementing it as an ordinary method, and then using a macro-style class method to register it as a callback. The last example could be written like that:

class User < ActiveRecord::Base
  validates_presence_of :login, :email

  before_validation :ensure_login_has_a_value

  protected
  def ensure_login_has_a_value
    if self.login.nil?
      self.login = email unless email.blank?
    end
  end
end

The macro-style class methods can also receive a block. Rails best practices say that you should only use this style of registration if the code inside your block is so short that it fits in just one line.

class User < ActiveRecord::Base
  validates_presence_of :login, :email

  before_create {|user| user.name = user.login.capitalize if user.name.blank?}
end

In Rails, the preferred way of registering callbacks is by using macro-style class methods. The main advantages of using macro-style class methods are:

You can add more than one method for each type of callback. Those methods will be queued for execution at the same order they were registered.
Readability, since your callback declarations will live at the beggining of your models' files.

Remember to always declare the callback methods as being protected or private. These methods should never be public, otherwise it will be possible to call them from code outside the model, violating object encapsulation and exposing implementation details.

9. Available callbacks

Here is a list with all the available Active Record callbacks, listed in the same order in which they will get called during the respective operations.

9.1. Callbacks called both when creating or updating a record.

before_validation
after_validation
before_save
INSERT OR UPDATE OPERATION
after_save

9.2. Callbacks called only when creating a new record.

before_validation_on_create
after_validation_on_create
before_create
INSERT OPERATION
after_create

9.3. Callbacks called only when updating an existing record.

before_validation_on_update
after_validation_on_update
before_update
UPDATE OPERATION
after_update

9.4. Callbacks called when removing a record from the database.

before_destroy
DELETE OPERATION
after_destroy

The before_destroy and after_destroy callbacks will only be called if you delete the model using either the destroy instance method or one of the destroy or destroy_all class methods of your Active Record class. If you use delete or delete_all no callback operations will run, since Active Record will not instantiate any objects, accessing the records to be deleted directly in the database.

9.5. The `after_initialize` and `after_find` callbacks

The after_initialize callback will be called whenever an Active Record object is instantiated, either by direcly using new or when a record is loaded from the database. It can be useful to avoid the need to directly override your Active Record initialize method.

The after_find callback will be called whenever Active Record loads a record from the database. When used together with after_initialize it will run first, since Active Record will first read the record from the database and them create the model object that will hold it.

The after_initialize and after_find callbacks are a bit different from the others, since the only way to register those callbacks is by defining them as methods. If you try to register after_initialize or after_find using macro-style class methods, they will just be ignored. This behaviour is due to performance reasons, since after_initialize and after_find will both be called for each record found in the database, significantly slowing down the queries.

10. Halting Execution

As you start registering new callbacks for your models, they will be queued for execution. This queue will include all your model's validations, the registered callbacks and the database operation to be executed. However, if at any moment one of the callback methods returns a boolean false (not nil) value, this execution chain will be halted and the desired operation will not complete: your model will not get persisted in the database, or your records will not get deleted and so on.

11. Callback classes

Sometimes the callback methods that you'll write will be useful enough to be reused at other models. Active Record makes it possible to create classes that encapsulate the callback methods, so it becomes very easy to reuse them.

Here's an example where we create a class with a after_destroy callback for a PictureFile model.

class PictureFileCallbacks
  def after_destroy(picture_file)
    File.delete(picture_file.filepath) if File.exists?(picture_file.filepath)
  end
end

When declared inside a class the callback method will receive the model object as a parameter. We can now use it this way:

class PictureFile < ActiveRecord::Base
  after_destroy PictureFileCallbacks.new
end

Note that we needed to instantiate a new PictureFileCallbacks object, since we declared our callback as an instance method. Sometimes it will make more sense to have it as a class method.

class PictureFileCallbacks
  def self.after_destroy(picture_file)
    File.delete(picture_file.filepath) if File.exists?(picture_file.filepath)
  end
end

If the callback method is declared this way, it won't be necessary to instantiate a PictureFileCallbacks object.

class PictureFile < ActiveRecord::Base
  after_destroy PictureFileCallbacks
end

You can declare as many callbacks as you want inside your callback classes.

12. Observers

Active Record callbacks are a powerful feature, but they can pollute your model implementation with code that's not directly related to the model's purpose. In object-oriented software, it's always a good idea to design your classes with a single responsability in the whole system. For example, it wouldn't make much sense to have a User model with a method that writes data about a login attempt to a log file. Whenever you're using callbacks to write code that's not directly related to your model class purposes, it may be a good moment to create an Observer.

An Active Record Observer is an object that links itself to a model and register it's methods for callbacks. Your model's implementation remain clean, while you can reuse the code in the Observer to add behaviuor to more than one model class. Ok, you may say that we can also do that using callback classes, but it would still force us to add code to our model's implementation.

Observer classes are subclasses of the ActiveRecord::Observer class. When this class is subclassed, Active Record will look at the name of the new class and then strip the Observer part to find the name of the Active Record class to observe.

Consider a Registration model, where we want to send an email everytime a new registration is created. Since sending emails is not directly related to our model's purpose, we could create an Observer to do just that:

class RegistrationObserver < ActiveRecord::Observer
  def after_create(model)
    # code to send registration confirmation emails...
  end
end

Like in callback classes, the observer's methods receive the observed model as a parameter.

Sometimes using the ModelName + Observer naming convention won't be the best choice, mainly when you want to use the same observer for more than one model class. It's possible to explicity specify the models that our observer should observe.

class Auditor < ActiveRecord::Observer
  observe User, Registration, Invoice
end

12.1. Registering observers

If you payed attention, you may be wondering where Active Record Observers are referenced in our applications, so they get instantiate and begin to interact with our models. For observers to work we need to register then in our application's config/environment.rb file. In this file there is a commented out line where we can define the observers that our application should load at start-up.

# Activate observers that should always be running
config.active_record.observers = :registration_observer, :auditor

12.2. Where to put the observers' source files

By convention, you should always save your observers' source files inside app/models.

13. Changelog

http://rails.lighthouseapp.com/projects/16213/tickets/26-active-record-validations-and-callbacks

Ruby on Rails

Sustainable productivity for web-application development

Chapters

Active Record Validations and Callbacks

1. Motivations to validate your Active Record objects

2. How it works

2.1. When does validation happens?

2.2. The meaning of valid

3. The declarative validation helpers

3.1. The validates_acceptance_of helper

3.2. The validates_associated helper

3.3. The validates_confirmation_of helper

3.4. The validates_each helper

3.5. The validates_exclusion_of helper

3.6. The validates_format_of helper

3.7. The validates_inclusion_of helper

3.8. The validates_length_of helper

3.9. The validates_numericallity_of helper

3.10. The validates_presence_of helper

3.11. The validates_uniqueness_of helper