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+h2. Active Record Validations and Callbacks
+
+This guide teaches you how to hook into 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 as well as how to perform custom operations at certain points in the object lifecycle.
+
+After reading this guide and trying out the presented concepts, we hope that you'll be able to:
+
+* Use the built-in Active Record validation helpers
+* Create your own custom validation methods
+* Work with the error messages generated by the validation process
+* Create callback methods to respond to events in the object lifecycle.
+* Create special classes that encapsulate common behavior for your callbacks
+* Create Rails Observers
+
+endprologue.
+
+# TODO consider starting with an overview of what validations and callbacks are, and the object lifecycle
+# http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
+# http://api.rubyonrails.org/classes/ActiveRecord/Base.html
+
+h3. Overview of ActiveRecord Validation
+
+Before you dive into the detail of validations in Rails, you should understand a bit about how validations fit into the big picture. Why should you use validations? When do these validations take place?
+
+h4. Why Use ActiveRecord Validations?
+
+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 that you could validate the data that goes to the database, including native database constraints, client-side validations, and model-level validations. Each of these 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 maintain 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. Additionally, database-level validations can safely handle some things (such as uniqueness in heavily-used tables) that are problematic to implement from the application level.
+* Implementing validations only at the client side can be difficult in 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 in 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 ability to easily create validations, providing built-in helpers for common validations while still allowing you to create your own validation methods.
+
+# TODO consider adding a bullet point on validations in controllers, and why model validations should be preferred over complicated controllers.
+# http://weblog.jamisbuck.org/2006/10/18/skinny-controller-fat-model
+
+h4. When Does Validation Happen?
+
+There are two kinds of Active Record objects: those that correspond to a row inside your database and those that 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 will be saved into the appropriate database table. Active Record uses the +new_record?+ instance method to determine whether an object is already in the database or not. Consider the following simple Active Record class:
+
+<ruby>
+class Person < ActiveRecord::Base
+end
+</ruby>
+
+We can see how it works by looking at some script/console output:
+
+<shell>
+>> 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
+</shell>
+
+Creating and saving a new record will send an SQL +INSERT+ operation to the database. Updating an existing record will send an SQL +UPDATE+ operation instead. Validations are typically run before these commands are sent to the database. If any validations fail, the object will be marked as invalid and Active Record will not trigger the +INSERT+ or +UPDATE+ operation. This helps to avoid storing an object in the database that's invalid. You can choose to have specific validations run when an object is created, saved, or updated.
+
+CAUTION: There are many ways to change the state of an object in the database. Some methods will trigger validations, but some will not. This means that it's possible to save an object in the database in an invalid state if you aren't careful.
+
+The following methods trigger validations, and will save the object to the database only if the object is valid. The bang versions (e.g. +save!+) will raise an exception if the record is invalid. The non-bang versions (e.g. +save+) simply return +false+.
+
+* +create+
+* +create!+
+* +save+
+* +save!+
+* +update+
+* +update_attributes+
+* +update_attributes!+
+
+h4. Skipping Validations
+
+The following methods skip validations, and will save the object to the database regardless of its validity. They should be used with caution.
+
+* +decrement!+
+* +decrement_counter+
+* +increment!+
+* +increment_counter+
+* +toggle!+
+* +update_all+
+* +update_attribute+
+* +update_counters+
+
+Note that +save+ also has the ability to skip validations (and callbacks!) if passed +false+. This technique should be used with caution.
+
+* +save(false)+
+
+h4. Object#valid? and Object#invalid?
+
+To verify whether or not an object is valid, you can use the +valid?+ method. This runs validations and returns true if no errors were added to the object, and false otherwise.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name
+end
+
+Person.create(:name => "John Doe").valid? # => true
+Person.create.valid? # => false
+</ruby>
+
+When Active Record is performing validations, any errors found are collected into an +errors+ instance variable and can be accessed through an +errors+ instance method. An object is considered invalid if it has errors, and calling +save+ or +save!+ will not save it to the database.
+
+However, note that an object instantiated with +new+ will not report errors even if it's technically invalid, because validations are not run when using +new+.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name
+end
+
+>> p = Person.new
+=> #<Person id: nil, name: nil>
+>> p.errors
+=> #<ActiveRecord::Errors:0x3b8b46c @base=#<Person id: nil, name: nil>, @errors={}>
+>> p.valid?
+=> false
+>> p.errors
+=> #<ActiveRecord::Errors:0x3b8b46c @base=#<Person id: nil, name: nil>, @errors={"name"=>["can't be blank"]}>
+>> p = Person.create
+=> #<Person id: nil, name: nil>
+>> p.errors
+=> #<ActiveRecord::Errors:0x3b8b46c @base=#<Person id: nil, name: nil>, @errors={"name"=>["can't be blank"]}>
+>> p.save
+=> false
+>> p.save!
+=> ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
+>> p = Person.create!
+=> ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
+</ruby>
+
+To verify whether or not a particular attribute of an object is valid, you can use the +invalid?+ method. This method is only useful _after_ validations have been run, because it only inspects the errors collection and does not trigger validations itself. It's different from the +valid?+ method because it doesn't verify the validity of the object as a whole, but only if there are errors found on an individual attribute of the object.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name
+end
+
+>> Person.new.errors.invalid?(:name) # => false
+>> Person.create.errors.invalid?(:name) # => true
+</ruby>
+
+h3. Declarative Validation Helpers
+
+Active Record offers many pre-defined validation helpers that you can use directly inside your class definitions. These helpers create validation rules that are commonly used. Every time a validation fails, an error message is added to the object's +errors+ collection, and this message is associated with the field being validated.
+
+Each helper accepts an arbitrary number of attributes identified by 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 of the values +:save+ (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.
+
+h4. validates_acceptance_of
+
+Validates that a checkbox on the user interface was checked when a form was submitted. This is normally used when the user needs to agree to your application's terms of service, confirm reading some text, 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).
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_acceptance_of :terms_of_service
+end
+</ruby>
+
+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 this.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_acceptance_of :terms_of_service, :accept => 'yes'
+end
+</ruby>
+
+h4. validates_associated
+
+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.
+
+<ruby>
+class Library < ActiveRecord::Base
+  has_many :books
+  validates_associated :books
+end
+</ruby>
+
+This validation will work with all the association types.
+
+CAUTION: Don't 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 each associated object will contain its own +errors+ collection; errors do not bubble up to the calling model.
+
+h4. validates_confirmation_of
+
+You should use this helper when you have two text fields that should receive exactly the same content. For example, you may want to confirm an email address or a password. This validation creates a virtual attribute, using the name of the field that has to be confirmed with '_confirmation' appended.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_confirmation_of :email
+end
+</ruby>
+
+In your view template you could use something like
+
+<erb>
+<%= text_field :person, :email %>
+<%= text_field :person, :email_confirmation %>
+</erb>
+
+NOTE: 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):
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_confirmation_of :email
+  validates_presence_of :email_confirmation
+end
+</ruby>
+
+The default error message for +validates_confirmation_of+ is "_doesn't match confirmation_"
+
+h4. validates_exclusion_of
+
+This helper validates that the attributes' values are not included in a given set. In fact, this set can be any enumerable object.
+
+<ruby>
+class MovieFile < ActiveRecord::Base
+  validates_exclusion_of :format, :in => %w(mov avi),
+    :message => "Extension %s is not allowed"
+end
+</ruby>
+
+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. This example uses the +:message+ option to show how you 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_".
+
+h4. validates_format_of
+
+This helper validates the attributes' values by testing whether they match a given pattern. This pattern must be specified using a Ruby regular expression, which is specified using the +:with+ option.
+
+<ruby>
+class Product < ActiveRecord::Base
+  validates_format_of :description, :with => /^[a-zA-Z]+$/,
+    :message => "Only letters allowed"
+end
+</ruby>
+
+The default error message for +validates_format_of+ is "_is invalid_".
+
+h4. validates_inclusion_of
+
+This helper validates that the attributes' values are included in a given set. In fact, this set can be any enumerable object.
+
+<ruby>
+class Coffee < ActiveRecord::Base
+  validates_inclusion_of :size, :in => %w(small medium large),
+    :message => "%s is not a valid size"
+end
+</ruby>
+
+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. The previous example uses the +:message+ option to show how you 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_".
+
+h4. validates_length_of
+
+This helper validates the length of your attribute's value. It includes a variety of different options, so you can specify length constraints in different ways:
+
+<ruby>
+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
+</ruby>
+
+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 constraint being used. You can still use the +:message+ option to specify an error message.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_length_of :bio, :too_long => "you're writing too much. %d characters is the maximum allowed."
+end
+</ruby>
+
+The +validates_size_of+ helper is an alias for +validates_length_of+.
+
+h4. validates_numericality_of
+
+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 set +:integer_only+ 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 to a number using +Kernel.Float+.
+
+<ruby>
+class Player < ActiveRecord::Base
+  validates_numericality_of :points
+  validates_numericality_of :games_played, :only_integer => true
+end
+</ruby>
+
+Besides +:only_integer+, the +validates_numericality_of+ helper also accepts the following options to add constraints to acceptable values:
+
+* +:greater_than+ - Specifies the value must be greater than the supplied value. The default error message for this option is "_must be greater than (value)_"
+* +:greater_than_or_equal_to+ - Specifies the value must be greater than or equal the supplied value. The default error message for this option is "_must be greater than or equal to (value)_"
+* +:equal_to+ - Specifies the value must be equal to the supplied value. The default error message for this option is "_must be equal to (value)_"
+* +:less_than+ - Specifies the value must be less than the supplied value. The default error message for this option is "_must e less than (value)_"
+* +:less_than_or_equal_to+ - Specifies the value must be less than or equal the supplied value. The default error message for this option is "_must be less or equal to (value)_"
+* +:odd+ - Specifies the value must be an odd number if set to true. The default error message for this option is "_must be odd_"
+* +:even+ - Specifies the value must be an even number if set to true. The default error message for this option is "_must be even_"
+
+
+The default error message for +validates_numericality_of+ is "_is not a number_".
+
+h4. validates_presence_of
+
+This helper validates that the specified 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).
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name, :login, :email
+end
+</ruby>
+
+NOTE: If you want to be sure that an association is present, you'll need to test whether the foreign key used to map the association is present, and not the associated object itself.
+
+<ruby>
+class LineItem < ActiveRecord::Base
+  belongs_to :order
+  validates_presence_of :order_id
+end
+</ruby>
+
+NOTE: If you want to validate the presence of a boolean field (where the real values are true and false), you should 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_".
+
+h4. validates_uniqueness_of
+
+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 intend to be unique. To avoid that, you must create an unique index in your database.
+
+<ruby>
+class Account < ActiveRecord::Base
+  validates_uniqueness_of :email
+end
+</ruby>
+
+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 are used to limit the uniqueness check:
+
+<ruby>
+class Holiday < ActiveRecord::Base
+  validates_uniqueness_of :name, :scope => :year,
+    :message => "Should happen once per year"
+end
+</ruby>
+
+There is also a +:case_sensitive+ option that you can use to define whether the uniqueness constraint will be case sensitive or not. This option defaults to true.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_uniqueness_of :name, :case_sensitive => false
+end
+</ruby>
+
+The default error message for +validates_uniqueness_of+ is "_has already been taken_".
+
+h4. validates_each
+
+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.
+
+<ruby>
+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
+</ruby>
+
+The block receives the model, the attribute's name and the attribute's value. You can do anything you like to check for valid data within the block. If your validation fails, you can add an error message to the model, therefore making it invalid.
+
+h3. 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 are discussed later in the conditional validation topic.
+
+h4. :allow_nil
+
+The +:allow_nil+ option skips the validation when the value being validated is +nil+. You may be asking yourself if it makes any sense to use +:allow_nil+ and +validates_presence_of+ together. Well, it does. Remember, the validation will be skipped only for +nil+ attributes, but empty strings are not considered +nil+.
+
+<ruby>
+class Coffee < ActiveRecord::Base
+  validates_inclusion_of :size, :in => %w(small medium large),
+    :message => "%s is not a valid size", :allow_nil => true
+end
+</ruby>
+
+h4. :allow_blank
+
+The +:allow_blank+ option is similar to the +:allow_nil+ option. This option will let validation pass if the attribute's value is +nil+ or an empty string, i.e., any value that returns +true+ for +blank?+.
+
+<ruby>
+class Topic < ActiveRecord::Base
+  validates_length_of :title, :is => 5, :allow_blank => true
+end
+
+Topic.create("title" => "").valid? # => true
+Topic.create("title" => nil).valid? # => true
+</ruby>
+
+h4. :message
+
+As you've already seen, 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, together with the attribute name.
+
+h4. :on
+
+The +:on+ option lets you specify when the validation should happen. The default behavior 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.
+
+<ruby>
+class Person < ActiveRecord::Base
+  # => it will be possible to update email with a duplicated value
+  validates_uniqueness_of :email, :on => :create
+
+  # => it will be possible to create the record with a 'non-numerical age'
+  validates_numericality_of :age, :on => :update
+
+  # => the default (validates on both create and update)
+  validates_presence_of :name, :on => :save
+end
+</ruby>
+
+h3. 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.
+
+h4. Using a symbol with :if and :unless
+
+You can associate 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.
+
+<ruby>
+class Order < ActiveRecord::Base
+  validates_presence_of :card_number, :if => :paid_with_card?
+
+  def paid_with_card?
+    payment_type == "card"
+  end
+end
+</ruby>
+
+h4. Using a string with the :if and :unless
+
+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.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :surname, :if => "name.nil?"
+end
+</ruby>
+
+h4. Using a Proc object with :if and :unless
+
+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 ability 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.
+
+<ruby>
+class Account < ActiveRecord::Base
+  validates_confirmation_of :password,
+    :unless => Proc.new { |a| a.password.blank? }
+end
+</ruby>
+
+h3. Writing your own validation methods
+
+When the built-in validation helpers are not enough for your needs, you can write your own validation methods. You can do that by implementing methods that verify the state of your models and add messages to their +errors+ collection when they are invalid. You must then register those methods by using one or more of the +validate+, +validate_on_create+ or +validate_on_update+ class methods, passing in the symbols for the validation methods' names. You can pass more than one symbol for each class method and the respective validations will be ran in the same order as they were registered.
+
+<ruby>
+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
+</ruby>
+
+You can even create your own validation helpers and reuse them in several different models. Here is an example where we create a custom validation helper to validate the format of fields that represent email addresses:
+
+<ruby>
+module ActiveRecord
+  module Validations
+    module ClassMethods
+      def validates_email_format_of(value)
+        validates_format_of value,
+          :with => /\A[\w\._%-]+@[\w\.-]+\.[a-zA-Z]{2,4}\z/,
+          :if => Proc.new { |u| !u.email.blank? },
+          :message => "Invalid format for email address"
+      end
+    end
+  end
+end
+</ruby>
+
+The recipe is simple: just create a new validation method inside the +ActiveRecord::Validations::ClassMethods+ module. You can put this code in a file inside your application's *lib* folder, and then requiring it from your *environment.rb* or any other file inside *config/initializers*. You can use this helper like this:
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_email_format_of :email_address
+end
+</ruby>
+
+h3. Manipulating the errors collection
+
+# TODO consider renaming section to something more simple, like "Validation errors"
+# Consider combining with new section at the top with Object#valid? and Object#invalid?
+# Consider moving this stuff above the current 2-6 sections (e.g. save the validation details until after this?)
+
+You can do more than just call +valid?+ upon your objects based on the existence 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.
+
+<ruby>
+class Person < ActiveRecord::Base
+  def a_method_used_for_validation_purposes
+    errors.add_to_base("This person is invalid because ...")
+  end
+end
+</ruby>
+
+* +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.
+
+<ruby>
+class Person < ActiveRecord::Base
+  def a_method_used_for_validation_purposes
+    errors.add(:name, "can't have the characters !@#$%*()_-+=")
+  end
+end
+</ruby>
+
+* +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.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name, :email
+end
+
+person = Person.new(:name => "John Doe")
+person.errors.invalid?(:email) # => true
+</ruby>
+
+* +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.
+
+<ruby>
+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)"]
+</ruby>
+
+* +clear+ is used when you intentionally want to clear all the messages in the +errors+ collection. However, calling +errors.clear+ upon an invalid object won't make it valid: the +errors+ collection will now be empty, but the next time you call +valid?+ or any method that tries to save this object to the database, the validations will run. If any of them fails, the +errors+ collection will get filled again.
+
+<ruby>
+class Person < ActiveRecord::Base
+  validates_presence_of :name
+  validates_length_of :name, :minimum => 3
+end
+
+person = Person.new
+person.valid? # => false
+person.errors.on(:name)
+# => ["can't be blank", "is too short (minimum is 3 characters)"]
+
+person.errors.clear
+person.errors.empty? # => true
+p.save # => false
+p.errors.on(:name)
+# => ["can't be blank", "is too short (minimum is 3 characters)"]
+</ruby>
+
+# TODO consider discussing other methods (e.g. errors.size)
+# http://api.rubyonrails.org/classes/ActiveRecord/Errors.html
+
+h3. Using error collection in views
+
+Rails provides built-in helpers to display the error messages of your models in your view templates. When creating a form with the form_for helper, you can use the error_messages method on the form builder to render all failed validation messages for the current model instance.
+
+<ruby>
+class Product < ActiveRecord::Base
+  validates_presence_of :description, :value
+  validates_numericality_of :value, :allow_nil => true
+end
+</ruby>
+
+<erb>
+<% form_for(@product) do |f| %>
+  <%= f.error_messages %>
+  <p>
+    <%= f.label :description %><br />
+    <%= f.text_field :description %>
+  </p>
+  <p>
+    <%= f.label :value %><br />
+    <%= f.text_field :value %>
+  </p>
+  <p>
+    <%= f.submit "Create" %>
+  </p>
+<% end %>
+</erb>
+
+!images/error_messages.png(Error messages)!
+
+You can also use the +error_messages_for+ helper to display the error messages of a model assigned to a view template. It's very similar to the previous example and will achieve exactly the same result.
+
+<erb>
+<%= error_messages_for :product %>
+</erb>
+
+The displayed text for each error message will always be formed by the capitalized name of the attribute that holds the error, followed by the error message itself.
+
+Both the +form.error_messages+ and the +error_messages_for+ helpers accept options that let you customize the +div+ element that holds the messages, changing the header text, the message below the header text and the tag used for the element that defines the header.
+
+<erb>
+<%= f.error_messages :header_message => "Invalid product!",
+  :message => "You'll need to fix the following fields:",
+  :header_tag => :h3 %>
+</erb>
+
+Which results in the following content
+
+!images/customized_error_messages.png(Customized error messages)!
+
+If you pass +nil+ to any of these options, it will get rid of the respective section of the +div+.
+
+It's also possible to change the CSS classes used by the +error_messages+ helper. These classes are automatically defined at the *scaffold.css* file, generated by the scaffold script. If you're not using scaffolding, you can still define those CSS classes at your CSS files. Here is a list of the default CSS classes.
+
+* +.fieldWithErrors+ - Style for the form fields with errors.
+* +#errorExplanation+ - Style for the +div+ element with the error messages.
+* +#errorExplanation h2+ - Style for the header of the +div+ element.
+* +#errorExplanation p+ - Style for the paragraph that holds the message that appears right below the header of the +div+ element.
+* +#errorExplanation ul li+ - Style for the list of error messages.
+
+h4. Changing the way form fields with errors are displayed
+
+By default, form fields with errors are displayed enclosed by a +div+ element with the +fieldWithErrors+ CSS class. However, we can write some Ruby code to override the way Rails treats those fields by default. Here is a simple example where we change the Rails behaviour to always display the error messages in front of each of the form fields with errors. The error messages will be enclosed by a +span+ element with a +validation-error+ CSS class. There will be no +div+ element enclosing the +input+ element, so we get rid of that red border around the text field. You can use the +validation-error+ CSS class to style it anyway you want.
+
+<ruby>
+ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
+  if instance.error_message.kind_of?(Array)
+    %(#{html_tag}<span class='validation-error'>&nbsp;
+      #{instance.error_message.join(',')}</span>)
+  else
+    %(#{html_tag}<span class='validation-error'>&nbsp;
+      #{instance.error_message}</span>)
+  end
+end
+</ruby>
+
+This will result in something like the following content:
+
+!images/validation_error_messages.png(Validation error messages)!
+
+The way form fields with errors are treated is defined by the +ActionView::Base.field_error_proc+ Ruby Proc. This Proc receives two parameters:
+
+* A string with the HTML tag
+* An object of the +ActionView::Helpers::InstanceTag+ class.
+
+h3. 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.
+
+# TODO discuss what does/doesn't trigger callbacks, like we did in the validations section (e.g. destroy versus delete).
+# Consider moving up to the (new) intro overview section, before getting into details.
+# http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M002220
+# http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
+
+h4. Callback registration
+
+In order to use the available callbacks, you need to register them. You can do that by implementing them as an ordinary methods, and then using a macro-style class method to register then as callbacks.
+
+<ruby>
+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
+</ruby>
+
+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.
+
+<ruby>
+class User < ActiveRecord::Base
+  validates_presence_of :login, :email
+
+  before_create {|user| user.name = user.login.capitalize if user.name.blank?}
+end
+</ruby>
+
+CAUTION: 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.
+
+# TODO consider not making this a warning, just a note (this is an implementation detail, not a requirement of the library)
+
+h3. Conditional callbacks
+
+Like in validations, we can also make our callbacks conditional, calling then only 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 callback *should* get called. If you want to specify when the callback *should not* be called, then you may use the +:unless+ option.
+
+h4. Using a symbol with the :if and :unless
+
+You can associate the +:if+ and +:unless+ options with a symbol corresponding to the name of a method that will get called right before the callback. If this method returns +false+ the callback won't be executed. This is the most common option. Using this form of registration it's also possible to register several different methods that should be called to check the if the callback should be executed.
+
+<ruby>
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number, :if => :paid_with_card?
+end
+</ruby>
+
+h4. Using a string with the :if and :unless
+
+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.
+
+<ruby>
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number, :if => "paid_with_card?"
+end
+</ruby>
+
+h4. Using a Proc object with the :if and :unless
+
+Finally, it's possible to associate +:if+ and +:unless+ with a Ruby Proc object. This option is best suited when writing short validation methods, usually one-liners.
+
+<ruby>
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number,
+    :if => Proc.new { |order| order.paid_with_card? }
+end
+</ruby>
+
+h4. Multiple Conditions for Callbacks
+
+When writing conditional callbacks, it's possible to mix both +:if+ and +:unless+ in the same callback declaration.
+
+<ruby>
+class Comment < ActiveRecord::Base
+  after_create :send_email_to_author, :if => :author_wants_emails?,
+    :unless => Proc.new { |comment| comment.post.ignore_comments? }
+end
+</ruby>
+
+h3. Available callbacks
+
+# TODO consider moving above the code examples and details, possibly combining with intro lifecycle stuff?
+
+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.
+
+h4. Callbacks called both when creating or updating a record.
+
+# TODO consider just listing these in the possible lifecycle of an object, roughly as they do here:
+# http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
+
+* +before_validation+
+* +after_validation+
+* +before_save+
+* *INSERT OR UPDATE OPERATION*
+* +after_save+
+
+h4. Callbacks called only when creating a new record.
+
+* +before_validation_on_create+
+* +after_validation_on_create+
+* +before_create+
+* *INSERT OPERATION*
+* +after_create+
+
+h4. Callbacks called only when updating an existing record.
+
+* +before_validation_on_update+
+* +after_validation_on_update+
+* +before_update+
+* *UPDATE OPERATION*
+* +after_update+
+
+h4. 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.
+
+h4. after_initialize and after_find callbacks
+
+The +after_initialize+ callback will be called whenever an Active Record object is instantiated, either by directly 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.
+
+h3. 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 +before_create+, +before_save+, +before_update+ or +before_destroy+ callback methods returns a boolean +false+ (not +nil+) value or raise and exception, 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. It's because the whole callback chain is wrapped in a transaction, so raising an exception or returning +false+ fires a database ROLLBACK.
+
+h3. 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.
+
+<ruby>
+class PictureFileCallbacks
+  def after_destroy(picture_file)
+    File.delete(picture_file.filepath) if File.exists?(picture_file.filepath)
+  end
+end
+</ruby>
+
+When declared inside a class the callback method will receive the model object as a parameter. We can now use it this way:
+
+<ruby>
+class PictureFile < ActiveRecord::Base
+  after_destroy PictureFileCallbacks.new
+end
+</ruby>
+
+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.
+
+<ruby>
+class PictureFileCallbacks
+  def self.after_destroy(picture_file)
+    File.delete(picture_file.filepath) if File.exists?(picture_file.filepath)
+  end
+end
+</ruby>
+
+If the callback method is declared this way, it won't be necessary to instantiate a PictureFileCallbacks object.
+
+<ruby>
+class PictureFile < ActiveRecord::Base
+  after_destroy PictureFileCallbacks
+end
+</ruby>
+
+You can declare as many callbacks as you want inside your callback classes.
+
+h3. 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 responsibility 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 registers its methods for callbacks. Your model's implementation remains clean, while you can reuse the code in the Observer to add behaviour 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 every time 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:
+
+<ruby>
+class RegistrationObserver < ActiveRecord::Observer
+  def after_create(model)
+    # code to send registration confirmation emails...
+  end
+end
+</ruby>
+
+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.
+
+<ruby>
+class Auditor < ActiveRecord::Observer
+  observe User, Registration, Invoice
+end
+</ruby>
+
+h4. Registering observers
+
+If you paid attention, you may be wondering where Active Record Observers are referenced in our applications, so they get instantiated and begin to interact with our models. For observers to work we need to register them somewhere. The usual place to do that is 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.
+
+<ruby>
+# Activate observers that should always be running
+config.active_record.observers = :registration_observer, :auditor
+</ruby>
+
+You can uncomment the line with +config.active_record.observers+ and change the symbols for the name of the observers that should be registered.
+
+It's also possible to register callbacks in any of the files living at *config/environments/*, if you want an observer to work only in a specific environment. There is not a +config.active_record.observers+ line at any of those files, but you can simply add it.
+
+h4. Where to put the observers' source files
+
+By convention, you should always save your observers' source files inside *app/models*.
+
+# TODO this probably doesn't need it's own section and entry in the table of contents.
+
+h3. Changelog
+
+"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213/tickets/26-active-record-validations-and-callbacks
+
+January 9, 2009: Initial version by "Cássio Marques":credits.html#cmarques