Activerecord Validations Readme

ActiveRecord can validate our models for us before they even touch the database. This means it's harder to end up with bad data, which can cause problems later even if our code is technically bug-free.

We can use ActiveRecord::Base helper methods like #validates to set things up.


After this lesson, you should be able to:

  • Identify when validation occurs in the lifespan of an object
  • Introspect on the ActiveRecord::Errors collection object on an AR instance
    • use #valid?
    • use #errors
  • Generate full_messages for errors
  • Check an attribute for validation errors
  • Add a custom validation error to an AR model

Context: Databases and Data Validity

What is a "validation"?

In the context of Rails, validations are special method calls that go at the top of model class definitions and prevent them from being saved to the database if their data doesn't look right.

In general, "validations" are any code that perform the job of protecting the database from invalid data.

AR Validations Are Not Database Constraints (Validations)

Many relational databases, such as SQLite and PostgreSQL, have data validation features that check things like length and data type. These validations are typically added via migrations, and depending on the specific validation, they may or may not be reflected in the schema.rb file.

Database constraints and model validations are also functionally different. Database constraints will ALWAYS be checked when adding or updating data in the database, while AR validations will only be checked when adding or updating data through Ruby/Rails (e.g. if we use SQL code in the command line to modify the database, AR validations are not run).

Some developers use database constraints and AR validations, while others rely on AR validations alone. Ultimately, it depends on how the developer plans to add and update data in the database. In this lesson, we'll be focusing on AR validations.

What is "invalid data"?

Suppose you get a new phone and you ask all of your friends for their phone number again. One of them tells you, "555-868-902". If you're paying attention, you'll probably wrinkle your nose and think, "Wait a minute. That doesn't sound like a real phone number."

"555-868-902" is an example of invalid data... for a phone number. It's probably a valid account number for some internet service provider in Alaska, but there's no way to figure out what your friend's phone number is from those nine numbers. It's a showstopper, and even worse, it kind of looks like valid data if you're not looking closely.

Validations Protect the Database

Invalid data is the bogeyman of web applications: it hides in your database until the worst possible moment, then jumps out and ruins everything by causing confusing errors.

Imagine the phone number above being saved to the database in an application that makes automatic calls using the Twilio API. When your system tries to call this number, there will be an error because no such phone number exists, which means you need to have an entire branch of code dedicated to handling just that edge case.

It would be much easier if you never have bad data in the first place, so you can focus on handling edge cases that are truly unpredictable.

That's where validations come in.

Basic Usage

For more examples of basic validation usage, see the Rails Guide for Active Record Validations. Take a few minutes to browse the helpers listed in Section 2.

class Person < ActiveRecord::Base
  validates :name, presence: true

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

#validates is our Swiss Army knife for validations. It takes two arguments: the first is the name of the attribute we want to validate, and the second is a hash of options that will include the details of how to validate it.

In this example, the options hash is { presence: true }, which implements the most basic form of validation, preventing the object from being saved if its name attribute is empty.

Lifecycle Timing

Before proceeding, keep the answer to this question in mind:

What is the difference between #new and #create?

If you've forgotten, #new instantiates a new ActiveRecord model without saving it to the database, whereas #create immediately attempts to save it, as if you had called #new and then #save.

Database activity triggers validation. An ActiveRecord model instantiated with #new will not be validated, because no attempt to write to the database was made. Validations won't run unless you call a method that actually hits the DB, like #save.

The only way to trigger validation without touching the database is to call the #valid? method.

For a full list of methods that trigger validation, see Section 4 of the Rails Guide for Active Record Callbacks. Don't worry about the rest of the information in that guide just yet; we'll go into callbacks later!

Validation Failure

Here it is, the moment of truth. What can we do when a record fails validation?

How can you tell when a record fails validation?

Pay attention to return values!

By default, ActiveRecord does not raise an exception when validation fails. DB operation methods (such as #save) will simply return false and decline to update the database.

Every database method has a sister method with a ! at the end which will raise an exception (#create! instead of #create and so on).

And of course, you can always check manually with #valid?.

class Person < ActiveRecord::Base
  validates :name, presence: true

person =
person.valid? #=> false #=> false! #=> EXCEPTION

Finding out why validations failed

To find out what went wrong, you can look at the model's #errors object.

You can check all errors at once by examining errors.messages.

person =
person.errors.messages #=> empty
person.valid? #=> false
person.errors.messages #=> name: can't be blank

You can check one attribute at a time by passing the name to errors as a key, like so:


Displaying Validation Errors in Views

See Section 8 of the Rails Guide for an example of how to use the ActiveModel::Errors#full_messages helper, reproduced here for convenience:

<% if @article.errors.any? %>
  <div id="error_explanation">
      <%= pluralize(@article.errors.count, "error") %>
      prohibited this article from being saved:

    <% @article.errors.full_messages.each do |msg| %>
      <li><%= msg %></li>
    <% end %>
<% end %>

This constructs more complete-looking sentences from the more terse messages available in errors.messages.

Other Built-in Validators

Rails has a host of built-in helpers.


length is one of the most versatile:

class Person < ActiveRecord::Base
  validates :name, length: { minimum: 2 }
  validates :bio, length: { maximum: 500 }
  validates :password, length: { in: 6..20 }
  validates :registration_number, length: { is: 6 }

The in argument makes use of a Range.

Remember that there's no syntactical magic happening with any of these method calls. If we weren't using Ruby's "poetry mode" (which is considered standard for Rails), the above code would look like this:

class Person < ActiveRecord::Base
  validates(:name, { :length => { :minimum => 2 } })
  validates(:bio, { :length => { :maximum => 500 } })
  validates(:password, { :length => { :in => 6..20 } })
  validates(:registration_number, { :length => { :is => 6 } })



Another common built-in validator is uniqueness:

class Account < ActiveRecord::Base
  validates :email, uniqueness: true

This will prevent any account from being created with the same email as another already-existing account.

Custom Messages

This isn't a validator in its own right, but a handy convenience option for specifying your own error messages:

class Person < ActiveRecord::Base
  validates :not_a_robot, acceptance: true, message: "Humans only!"

Custom Validators

There are three ways to implement custom validators, with examples in Section 6 of the Rails Guide.

Of the three, #validate is the simplest. If your validation needs become more complex, consult the documentation. For most validations, though, the following method should be good enough.

  1. Create a new directory in app called validators. Because most Rails developers don't need to write custom validation, this directory is not created by default like models or controllers.
  2. Identify the ActiveRecord attribute you want to validate. Is it the email or the last_name on the Person class, for example?
  3. Create a new file in the app/validators directory of the form attribute (from the previous step) + _validator.rb. So in the case of validating an attribute called email, create a file app/validators/email_validator.rb
  4. Inside the new file, define the class. The class name should match the file name of the file, but "Camel-Cased." So email_validator should be class EmailValidator. The class should inherit from ActiveModel::Validator
  5. The validator class must have one instance method, #validate. This method will receive one argument typically called record.
  6. Inside of #validate, you'll be able to get properties from record and determine whether it is invalid. If the record is invalid, push (<<) to record.errors[:attribute] e.g. record.errors[:email] a String which is a message that you want to display that describes why the message is not valid.
  7. Lastly, in the implementation of the class being validated e.g. Person, add:
    1. An include of ActiveModel::Validations
    2. The helper call: validates_with (className). In our example we'd put, validates_with EmailValidator (see step 4, above)

The result of these steps should be the following:

class EmailValidator < ActiveModel::Validator
  def validate(record)
      record.errors[:email] << "We're only allowed to have people who work for the company in the database!"
class Person
  include ActiveModel::Validations
  validates_with EmailValidator

Here we validate that all email addresses are in the domain.

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