custom-validations.md

Validations II: Custom Validations

When the built-in validation helpers are not enough for your needs, you can write your own validation methods or validator classes.

Custom Methods

With validation methods, you can write your own methods to validate a model.

class Invoice < ApplicationRecord
  # validate tells the class to run these methods at validation time.
  validate :discount_cannot_be_greater_than_total_value

  private
  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors[:discount] << 'can\'t be greater than total value'
    end
  end
end

Note that the presence of an error is registered by adding a message to the errors hash. If no error messages are added, the validation is deemed to have passed. Note that the message does not mention the variable name; when you call full_messages, Rails will prefix the message with the attribute name for you.

errors[:base]

Sometimes an error is not specific to any one attribute. In this case, you add the error to errors[:base]. Since errors[:base] is an array, you can simply add a string to it and it will be used as an error message.

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors[:base] << 'This person is invalid because ...'
  end
end

Custom Validators

Custom validators are classes that extend ActiveModel::EachValidator. Prefer writing a custom validator class when you want to reuse validation logic for multiple models or multiple columns. Otherwise, it's simpler to use a validator method.

Your custom validator class must implement a validate_each method which takes three arguments: the record, the attribute name and its value.

# app/validators/email_validator.rb
class EmailValidator < ActiveModel::EachValidator
  CRAZY_EMAIL_REGEX = /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i

  def validate_each(record, attribute_name, value)
    unless value =~ CRAZY_EMAIL_REGEX
      # we can use `EachValidator#options` to access custom options
      # passed to the validator.
      message = options[:message] || 'is not an email'
      record.errors[attribute_name] << message
    end
  end
end

# app/models/person.rb
class Person < ApplicationRecord
  # Rails knows `:email` means `EmailValidator`.
  validates :email, presence: true, email: true
  # not required, but must also be an email
  validates :backup_email, email: { message: 'isn\'t even valid' }
end

As shown in the example, you can also combine standard validations with your own custom validators.