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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.

  1. TODO consider starting with an overview of what validations and callbacks are, and the object lifecycle
  2. http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
  3. http://api.rubyonrails.org/classes/ActiveRecord/Base.html

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?

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.
  1. TODO consider adding a bullet point on validations in controllers, and why model validations should be preferred over complicated controllers.
  2. http://weblog.jamisbuck.org/2006/10/18/skinny-controller-fat-model

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:

class Person < ActiveRecord::Base
end

We can see how it works by looking at some 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

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!

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)

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.

class Person < ActiveRecord::Base
validates_presence_of :name
end

Person.create(:name => “John Doe”).valid? # => true
Person.create.valid? # => false

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.

class Person < ActiveRecord::Base
validates_presence_of :name
end

>> p = Person.new
=> #<Person id: nil, name: nil>
>> p.errors
=> #<ActiveRecord::Errors:0×3b8b46c @base=#<Person id: nil, name: nil>, @errors={}>
>> p.valid?
=> false
>> p.errors
=> #<ActiveRecord::Errors:0×3b8b46c @base=#<Person id: nil, name: nil>, @errors={"name"=>[“can’t be blank”]}>
>> p = Person.create
=> #<Person id: nil, name: nil>
>> p.errors
=> #<ActiveRecord::Errors:0×3b8b46c @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

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.

class Person < ActiveRecord::Base
validates_presence_of :name
end

>> Person.new.errors.invalid?(:name) # => false
>> Person.create.errors.invalid?(:name) # => true

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.

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).

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 this.

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

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.

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

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.

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.

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 %>

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):

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

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.

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. 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”.

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.

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”.

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.

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. 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”.

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:

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 constraint 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

The validates_size_of helper is an alias for validates_length_of.

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.

class Player < ActiveRecord::Base
validates_numericality_of :points
validates_numericality_of :games_played, :only_integer => true
end

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”.

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).

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

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.

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

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”.

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.

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 are used to limit the uniqueness check:

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 whether the uniqueness constraint 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”.

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.

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. 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.

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.

: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.

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

: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?.

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

: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.

: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.

class Person < ActiveRecord::Base

  1. => it will be possible to update email with a duplicated value
    validates_uniqueness_of :email, :on => :create
  1. => it will be possible to create the record with a ‘non-numerical age’
    validates_numericality_of :age, :on => :update
  1. => the default (validates on both create and update)
    validates_presence_of :name, :on => :save
    end

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.

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.

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

def paid_with_card? payment_type == “card” end

end

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.

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

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.

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

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.

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

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:

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

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:

class Person < ActiveRecord::Base
validates_email_format_of :email_address
end

Manipulating the errors collection

  1. TODO consider renaming section to something more simple, like “Validation errors”
  2. Consider combining with new section at the top with Object#valid? and Object#invalid?
  3. 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.

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.errors.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)

  1. => “is too short (minimum is 3 characters)”

person = Person.new
person.valid? # => false
person.errors.on(:name)

  1. => [“can’t be blank”, “is too short (minimum is 3 characters)”]
  • 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.

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)

  1. => [“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)

  1. => [“can’t be blank”, “is too short (minimum is 3 characters)”]
  1. TODO consider discussing other methods (e.g. errors.size)
  2. http://api.rubyonrails.org/classes/ActiveRecord/Errors.html

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.

class Product < ActiveRecord::Base
validates_presence_of :description, :value
validates_numericality_of :value, :allow_nil => true
end

<% form_for(@product) do |f| >
<= f.error_messages >


<

= f.label :description >
<= f.text_field :description %>

<%= f.label :value %>
<%= f.text_field :value %>

<%= f.submit “Create” %>

<% end %>

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.

<%= error_messages_for :product %>

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.

<%= f.error_messages :header_message => “Invalid product!”,
:message => “You’ll need to fix the following fields:”,
:header_tag => :h3 %>

Which results in the following content

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.

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.

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
if instance.error_message.kind_of?(Array)
%(#{html_tag} 
#{instance.error_message.join(‘,’)})
else
%(#{html_tag} 
#{instance.error_message})
end
end

This will result in something like the following content:

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.

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.

  1. TODO discuss what does/doesn’t trigger callbacks, like we did in the validations section (e.g. destroy versus delete).
  2. Consider moving up to the (new) intro overview section, before getting into details.
  3. http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M002220
  4. http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html

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.

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

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.

  1. TODO consider not making this a warning, just a note (this is an implementation detail, not a requirement of the library)

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.

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.

class Order < ActiveRecord::Base
before_save :normalize_card_number, :if => :paid_with_card?
end

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.

class Order < ActiveRecord::Base
before_save :normalize_card_number, :if => “paid_with_card?”
end

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.

class Order < ActiveRecord::Base
before_save :normalize_card_number,
:if => Proc.new { |order| order.paid_with_card? }
end

Multiple Conditions for Callbacks

When writing conditional callbacks, it’s possible to mix both :if and :unless in the same callback declaration.

class Comment < ActiveRecord::Base
after_create :send_email_to_author, :if => :author_wants_emails?,
:unless => Proc.new { |comment| comment.post.ignore_comments? }
end

Available callbacks

  1. 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.

Callbacks called both when creating or updating a record.

  1. TODO consider just listing these in the possible lifecycle of an object, roughly as they do here:
  2. http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
  • before_validation
  • after_validation
  • before_save
  • INSERT OR UPDATE OPERATION
  • after_save

Callbacks called only when creating a new record.

  • before_validation_on_create
  • after_validation_on_create
  • before_create
  • INSERT OPERATION
  • after_create

Callbacks called only when updating an existing record.

  • before_validation_on_update
  • after_validation_on_update
  • before_update
  • UPDATE OPERATION
  • after_update

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.

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.

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.

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.

Observers

Observers are similar to callbacks, but with important differences. Whereas callbacks can pollute a model with code that isn’t directly related to its purpose, observers allow you to add functionality outside of a model. For example, it could be argued that a User model should not include code to send registration confirmation emails. Whenever you use callbacks with code that isn’t directly related to your model, you may want to consider creating an observer instead.

Creating observers

For example, imagine a User model where we want to send an email every time a new user is created. Because sending emails is not directly related to our model’s purpose, we could create an observer to contain this functionality.

class UserObserver < ActiveRecord::Observer
def after_create(model)

  1. code to send confirmation email…
    end
    end

As with callback classes, the observer’s methods receive the observed model as a parameter.

Registering observers

Observers should be placed inside of your app/models directory and registered in your application’s config/environment.rb file. For example, the UserObserver above would be saved as app/models/user_observer.rb and registered in config/environment.rb.

  1. Activate observers that should always be running
    config.active_record.observers = :user_observer

As usual, settings in config/environments/ take precedence over those in config/environment.rb. So, if you prefer that an observer not run in all environments, you can simply register it in a specific environment instead.

Sharing observers

By default, Rails will simply strip ‘observer’ from an observer’s name to find the model it should observe. However, observers can also be used to add behaviour to more than one model, and so it’s possible to manually specify the models that our observer should observe.

class MailerObserver < ActiveRecord::Observer
observe :registration, :user

def after_create(model)
  1. code to send confirmation email…
    end
    end

In this example, the after_create method would be called whenever a Registration or User was created. Note that this new MailerObserver would also need to be registered in config/environment.rb in order to take effect.

  1. Activate observers that should always be running
    config.active_record.observers = :mailer_observer

Changelog

Lighthouse ticket

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