This guide teaches you how to hook into the lifecycle of your Active Record objects. You will learn how to validate the state of objects before they go into the database, and how to perform custom operations at certain points in the object lifecycle.
During the normal operation of a Rails application objects may be created, updated, and destroyed. Active Record provides hooks into this <em>object lifecycle</em> so that you can control your application and its data.
Validations allow you to ensure that only valid data is stored in your database. Callbacks and observers allow you to trigger logic before or after an alteration of an object's state.
Validations are used to ensure that only valid data is saved into your database. For example, it may be important to your application to ensure that every user provides a valid email address and mailing address.
There are several ways to validate data before it is saved into your database, including native database constraints, client-side validations, controller-level validations, and model-level validations.
* Database constraints and/or stored procedures make the validation mechanisms database-dependent and can make testing and maintenance more difficult. However, if your database is used by other applications, it may be a good idea to use some constraints at the database level. Additionally, database-level validations can safely handle some things (such as uniqueness in heavily-used tables) that can be difficult to implement otherwise.
* Client-side validations can be useful, but are generally unreliable if used alone. If they are implemented using JavaScript, they may be bypassed if JavaScript is turned off in the user's browser. However, if combined with other techniques, client-side validation can be a convenient way to provide users with immediate feedback as they use your site.
* Controller-level validations can be tempting to use, but often become unwieldy and difficult to test and maintain. Whenever possible, it's a good idea to "keep your controllers skinny":http://weblog.jamisbuck.org/2006/10/18/skinny-controller-fat-model, as it will make your application a pleasure to work with in the long run.
* Model-level validations are the best way to ensure that only valid data is saved into your database. They are database agnostic, cannot be bypassed by end users, and are convenient to test and maintain. Rails makes them easy to use, provides built-in helpers for common needs, and allows you to create your own validation methods as well.
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, for example 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:
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 perform the +INSERT+ or +UPDATE+ operation. This helps to avoid storing an invalid object in the database. 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 bang versions (e.g. +save!+) raise an exception if the record is invalid. The non-bang versions don't: +save+ and +update_attributes+ return +false+, +create+ and +update+ just return the object/s.
To verify whether or not an object is valid, Rails uses the +valid?+ method. You can also use this method on your own. +valid?+ triggers your validations and returns true if no errors were added to the object, and false otherwise.
When Active Record is performing validations, any errors found can be accessed through the +errors+ instance method. By definition an object is valid if this collection is empty after running validations.
+invalid?+ is simply the inverse of +valid?+. +invalid?+ triggers your validations and returns true if any errors were added to the object, and false otherwise.
To verify whether or not a particular attribute of an object is valid, you can use +errors[:attribute]+ that returns an array with all attribute errors, when there are no errors on the specified attribute, an empty array is returned.
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 +ActiveRecord::Base#invalid?+ method explained above because it doesn't verify the validity of the object as a whole. It only checks to see whether there are errors found on an individual attribute of the object.
We'll cover validation errors in greater depth in the "Working with Validation Errors":#working-with-validation-errors section. For now, let's turn to the built-in validation helpers that Rails provides by default.
Active Record offers many pre-defined validation helpers that you can use directly inside your class definitions. These helpers provide common validation rules. 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 attribute names, so with a single line of code you can add the same kind of validation to several attributes.
All of them accept the +:on+ and +:message+ options, which define when the validation should be run and what message should be added to the +errors+ collection if 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 specified. Let's take a look at each one of the available helpers.
Validates that a checkbox on the user interface was checked when a form was submitted. This is typically 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 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).
+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.
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.
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.
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 whose name is the name of the field that has to be confirmed with "_confirmation" appended.
This check is performed only if +email_confirmation+ is not +nil+. 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):
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 include the attribute's value.
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 include the attribute's 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 <tt>%{count}</tt> 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.
This helper validates that your attributes have only numeric values. By default, it will match an optional sign followed by an integral or floating point number. To specify that only integral numbers are allowed set +:only_integer+ to true.
* +:greater_than+ - Specifies the value must be greater than the supplied value. The default error message for this option is "_must be greater than %{count}_".
* +:greater_than_or_equal_to+ - Specifies the value must be greater than or equal to the supplied value. The default error message for this option is "_must be greater than or equal to %{count}_".
* +:equal_to+ - Specifies the value must be equal to the supplied value. The default error message for this option is "_must be equal to %{count}_".
* +:less_than+ - Specifies the value must be less than the supplied value. The default error message for this option is "_must be less than %{count}_".
* +: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 %{count}_".
This helper validates that the specified attributes are not empty. It uses the +blank?+ method to check if the value is either +nil+ or a blank string, that is, a string that is either empty or consists of whitespace.
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.
Since +false.blank?+ is true, if you want to validate the presence of a boolean field you should use +validates_inclusion_of :field_name, :in => [true, false]+.
This helper validates that the attribute's value is unique right before the object gets saved. It does not create a uniqueness constraint in the 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.
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.
This helper passes the record to a separate class for validation.
<ruby>
class Person < ActiveRecord::Base
validates_with GoodnessValidator
end
class GoodnessValidator < ActiveRecord::Validator
def validate
if record.first_name == "Evil"
record.errors[:base] << "This person is evil"
end
end
end
</ruby>
The +validates_with+ helper takes a class, or a list of classes to use for validation. There is no default error message for +validates_with+. You must manually add errors to the record's errors collection in the validator class.
The validator class has two attributes by default:
* +record+ - the record to be validated
* +options+ - the extra options that were passed to +validates_with+
Like all other validations, +validates_with+ takes the +:if+, +:unless+ and +:on+ options. If you pass any other options, it will send those options to the validator class as +options+:
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|
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.
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 "Conditional Validation":#conditional-validation.
The +:allow_nil+ option skips the validation when the value being validated is +nil+. Using +:allow_nil+ with +validates_presence_of+ allows for +nil+, but any other +blank?+ value will still be rejected.
The +:allow_blank+ option is similar to the +:allow_nil+ option. This option will let validation pass if the attribute's value is +blank?+, like +nil+ or an empty string for example.
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.
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.
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 +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.
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.
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.
Finally, it's possible to associate +:if+ and +:unless+ with a +Proc+ object which will be called. Using a +Proc+ object gives you the ability to write an inline condition instead of a separate method. This option is best suited for one-liners.
When the built-in validation helpers are not enough for your needs, you can write your own validation methods.
Simply create methods that verify the state of your models and add messages to the +errors+ collection when they are invalid. You must then register these 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 even create your own validation helpers and reuse them in several different models. For example, an application that manages surveys may find useful to express that a certain field corresponds to a set of choices:
Simply reopen +ActiveRecord::Base+ and define a class method like that. You'd typically put this code somewhere in +config/initializers+. You can use this helper like this:
In addition to the +valid?+ and +invalid?+ methods covered earlier, Rails provides a number of methods for working with the +errors+ collection and inquiring about the validity of objects.
The following is a list of the most commonly used methods. Please refer to the +ActiveRecord::Errors+ documentation for a list of all the available methods.
+errors[]+ is used when you want to check the error messages for a specific attribute. It returns an array of strings with all error messages for the given attribute, each string with one error message. If there are no errors related to the attribute returns an empty array.
The +add+ method lets you manually add messages that are related to particular attributes. You can use the +errors.full_messages+ or +errors.to_a+ methods to view the messages in the form they might be displayed to a user. Those particular messages get the attribute name prepended (and capitalized). +add+ receives the name of the attribute you want to add the message to, and the message itself.
# => ["cannot contain the characters !@#%*()_-+="]
person.errors.to_a
# => ["Name cannot contain the characters !@#%*()_-+="]
</ruby>
h4. +errors[:base]+
You can 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 its attributes. Since +errors[:base]+ is an array, you can simply add a string to the array and uses it as the error message.
The +clear+ method is used when you intentionally want to clear all the messages in the +errors+ collection. Of course, calling +errors.clear+ upon an invalid object won't actually 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 again. If any of the validations fail, the +errors+ collection will be filled again.
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.
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.
By default, form fields with errors are displayed enclosed by a +div+ element with the +fieldWithErrors+ CSS class. However, it's possible to override that.
The way form fields with errors are treated is defined by +ActionView::Base.field_error_proc+. This is a +Proc+ that receives two parameters:
* A string with the HTML tag
* An instance of +ActionView::Helpers::InstanceTag+.
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.
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, validated, or loaded from the database.
In order to use the available callbacks, you need to register them. You can do that by implementing them as ordinary methods, and then using a macro-style class method to register them as callbacks.
The macro-style class methods can also receive a block. Consider using this style if the code inside your block is so short that it fits in just one line.
It's considered good practice to declare callback methods as being protected or private. If left public, they can be called from outside of the model and violate the principle of object encapsulation.
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:
WARNING. +after_save+ runs both on create and update, but always _after_ the more specific callbacks +after_create+ and +after_update+, no matter the order in which the macro calls were executed.
h4. +after_initialize+ and +after_find+
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. +after_find+ is called before +after_initialize+ if both are defined.
The +after_initialize+ and +after_find+ callbacks are a bit different from the others. They have no +before_*+ counterparts, and the only way to register them is by defining them as regular 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.
<ruby>
class User < ActiveRecord::Base
def after_initialize
puts "You have initialized an object!"
end
def after_find
puts "You have found an object!"
end
end
>> User.new
You have initialized an object!
=> #<User id: nil>
>> User.first
You have found an object!
You have initialized an object!
=> #<User id: 1>
</ruby>
h3. Running Callbacks
The following methods trigger callbacks:
* +create+
* +create!+
* +decrement!+
* +destroy+
* +destroy_all+
* +increment!+
* +save+
* +save!+
* +save(false)+
* +toggle!+
* +update+
* +update_attribute+
* +update_attributes+
* +update_attributes!+
* +valid?+
Additionally, the +after_find+ callback is triggered by the following finder methods:
* +all+
* +first+
* +find+
* +find_all_by_<em>attribute</em>+
* +find_by_<em>attribute</em>+
* +find_by_<em>attribute</em>!+
* +last+
The +after_initialize+ callback is triggered every time a new object of the class is initialized.
h3. Skipping Callbacks
Just as with validations, it's also possible to skip callbacks. These methods should be used with caution, however, because important business rules and application logic may be kept in callbacks. Bypassing them without understanding the potential implications may lead to invalid data.
* +decrement+
* +decrement_counter+
* +delete+
* +delete_all+
* +find_by_sql+
* +increment+
* +increment_counter+
* +toggle+
* +update_all+
* +update_counters+
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.
The whole callback chain is wrapped in a transaction. If any before callback method returns exactly +false+ or raises an exception the execution chain gets halted and a ROLLBACK is issued. After callbacks can only accomplish that by raising an exception.
WARNING. Raising an arbitrary exception may break code that expects +save+ and friends not to fail like that. The +ActiveRecord::Rollback+ exception is thought precisely to tell Active Record a rollback is going on. That one is internally captured but not reraised.
h3. Relational Callbacks
Callbacks work through model relationships, and can even be defined by them. Let's take an example where a user has many posts. In our example, a user's posts should be destroyed if the user is destroyed. So, we'll add an +after_destroy+ callback to the +User+ model by way of its relationship to the +Post+ model.
Like in validations, we can also make our callbacks conditional, calling them 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 +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.
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 if the callback should be executed.
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.
Finally, it's possible to associate +:if+ and +:unless+ with a +Proc+ object. This option is best suited when writing short validation methods, usually one-liners.
Sometimes the callback methods that you'll write will be useful enough to be reused by other models. Active Record makes it possible to create classes that encapsulate the callback methods, so it becomes very easy to reuse them.
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.
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 the same 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.
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.
Observers are conventionally 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+ this way:
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.
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.
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.
