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?

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:

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:

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

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

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

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 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).

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.

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.

This validation will work with all the association types.

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, using the name of the field that has to be confirmed with _confirmation appended.

In your view template you could use something like

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

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

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".

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.

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

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

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".

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:

The possible length constraint options are:

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.

The validates_size_of helper is an alias for validates_length_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.

Besides :only_integer, the validates_numericality_of helper also accepts the following options to add constraints to acceptable values:

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

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).

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

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.

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:

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.

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

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.

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 the conditional validation topic.

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.

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?.

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.

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 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.

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

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.

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:

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:

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

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.

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.

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.

Which results in the following content

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.

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.

This will result in something like the following content:

The way form fields with errors are treated is defined by the ActionView::Base.field_error_proc Ruby Proc. This Proc receives two parameters:

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.

In order to use the available callbacks, you need to registrate 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.

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.

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.

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.

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 Ruby Proc object. This option is best suited when writing short validation methods, usually one-liners.

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

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.

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.

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

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

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

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.

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.

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

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.

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

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

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:

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.

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.

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.

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

Lighthouse ticket

January 9, 2009: Initial version by Cássio Marques