Rails Model Validations

Objectives

By the end of this lesson you should be able to...

• Know, what is validation and why we need it
• Add validations to our models

Introduction

Active Record allows you to validate the state of a model before it gets written into the database. There are several methods that you can use to check your models and validate that an attribute value is not empty, is unique and not already in the database, follows a specific format and many more.

Validation is a very important issue to consider when persisting to database, so the methods create, save and update take it into account when running: they return false when validation fails and they didn't actually perform any operation on database. All of these have a bang counterpart (that is, create!, save! and update!), which are stricter in that they raise the exception ActiveRecord::RecordInvalid if validation fails.

Let's look at how we might validate some things.

• Empty Values

  To prevent empty values we'll use validates <property>, presence: true. This is a slightly more restrictive check than null: false; it disallows both empty values (nil in Ruby, NULL in the database), and it also disallows empty strings (for string properties).

  EXAMPLE : To set an empty-value validator in our Country model, you might write

  class Country < ActiveRecord::Base
    has_many :citizenships
    has_many :people, through: :citizenships
  
    validates :name, presence: true
  end

• Uniqueness

  To ensure that a property is unique, we'll use validates <property>, uniqueness: true.

  EXAMPLE : To set some uniqueness validators in my Person model, you might write

  class Person < ActiveRecord::Base
    has_many :citizenships
    has_many :countries, through: :citizenships
  
    validates :phone_number, uniqueness: true
  end

• References

  For referential integrity checks, we'll use validates <model>, presence: true, where <model> is the symbol we passed to belongs_to.

  EXAMPLE : You want to test that a Citizenship instance refers to an instance of Country and an instance of Person; in that case, you might write the following:

  class Citizenship < ActiveRecord::Base
    belongs_to :country
    belongs_to :person
  
    validates :country, presence: true
    validates :person, presence: true
  end

ActiveRecord::Base comes with a slew of other validators we can use, as well as the mechanisms to create our own custom validators.

A quick example to illustrate:

class Person < ActiveRecord::Base
  validates :first_name, presence: true
end

person = Person.new(last_name: "Bashir")
person.valid?  # => checks validations and returns a boolean
person.save    # => rollback db
person.save!   # => specifies failure

person.first_name = "Usman"
person.valid?
person.save

You can learn more about validations in the Active Record Validations guide.

Validate Person Model

We set our validations in our app/models/person.rb model.

Let's make sure each person definitely has a first name and a last name before they're saved.

class Person < ActiveRecord::Base
  validates :first_name, presence: true
  validates :last_name, presence: true
end

• Type reload! into the console to update your model validations.
• Try saving a person with no first or last name and see what error is thrown.
• Try calling valid? on a new person.
• Walk through the Rails Docs and show that you can add error messages to your validations (e.g.- too_short: "must have at least %{count} words").

WE DO:

Add the following validation to person.rb:

validates :hometown, :first_name, :last_name, length: { minimum: 3, too_short: "must have at least %{count} words" }

or add numericality to the age field:

validates :age, numericality: true

YOU DO:

• Create a new Person table column named age and add a validation.
• Attempt to create a new Person and make it fail.
• Make the new Person pass.

Migrations/Validations Exercise

Can be found here

Further Reading

• Active Record Overview - Rails Guides
• Active Record Migrations - Rails Guides
• Active Record Associations - Rails Guides
• Active Record Validations - Rails Guides