Extract Validator

Extract Validator is a form of extract class that is used to remove complex validation details from ActiveRecord models. This technique also prevents duplication of validation code across several files.

Uses

• Keeps validation implementation details out of models.
• Encapsulates validation details into a single file, following the single responsibility principle.
• Removes duplication among classes performing the same validation logic.
• Makes validation logic easier to reuse, which makes it easier to avoid duplication.

Example

The Invitation class has validation details in-line. It checks that the recipient_email matches the formatting of the regular expression EMAIL_REGEX.

# app/models/invitation.rb
class Invitation < ActiveRecord::Base
  EMAIL_REGEX = /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i
  validates :recipient_email, presence: true, format: EMAIL_REGEX
end

We extract the validation details into a new class EmailValidator and place the new class into the app/validators directory:

# app/validators/email_validator.rb
class EmailValidator < ActiveModel::EachValidator
  EMAIL_REGEX = /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i
  def validate_each(record, attribute, value)
    unless value.match EMAIL_REGEX
      record.errors.add(attribute, "#{value} is not a valid email")
    end
  end
end

Once the validator has been extracted, Rails has a convention for using the new validation class. EmailValidator is used by setting email: true in the validation arguments:

# app/models/invitation.rb
class Invitation < ActiveRecord::Base
  validates :recipient_email, presence: true, email: true
end

The convention is to use the validation class name (in lower case, and removing Validator from the name). For example, if we were validating an attribute with ZipCodeValidator, we’d set zip_code: true as an argument to the validation call.

When validating an array of data as we do in SurveyInviter, we use the EnumerableValidator to loop over the contents of an array.

# app/models/survey_inviter.rb
validates_with EnumerableValidator,
  attributes: [:recipients],
  unless: 'recipients.nil?',
  validator: EmailValidator

The EmailValidator is passed in as an argument, and each element in the array is validated against it.

# app/validators/enumerable_validator.rb
class EnumerableValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, enumerable)
    enumerable.each do |value|
      validator.validate_each(record, attribute, value)
    end
  end

  private

  def validator
    options[:validator].new(validator_options)
  end

  def validator_options
    options.except(:validator).merge(attributes: attributes)
  end
end

Please note that in the latest version of the example application the EmailValidator class was renamed to EmailAddressValidator to avoid a naming conflict with an external gem.

Next Steps

• Verify the extracted validator does not have any long methods.
• Check for other models that could use the validator.