Dogweather

Robert Shecter

Self-validating Ruby objects with ActiveModel Validations

I’m importing lots of CSV restaurant inspection data with Ruby, and I need to make sure the cleaned up data matches the spec. For example, a violation must have a business_id and date. It can optionally have a code and description. My goal was to be able to write a class like this:

class Violation < ValidatedObject
  attr_accessor :business_id, :date, :code, :description

  validates :business_id, presence: true
  validates :date, presence: true, type: Date
end

…and it would just work, Rails-style:

Violation.new do |v|
  v.business_id = '1234'
  v.date = '2015-01-15'
end
# => ArgumentError: date must be of the class Date

Violation.new do |v|
 v.date = Date.new(2015, 1, 25)
end
# => ArgumentError: business_id is required

Violation.new do |v|
 v.business_id = '1234'
 v.date = Date.new(2015, 1, 25)
end
# => new instance Violation<...>

ActiveModel::Validations was refactored out of ActiveRecord to enable just this sort of use case. This is because the awesome list of built-in validations (here and here) and methods like #valid? are useful in a variety of contexts, not just in Rails.

It turned out that it was easy to write a small base class with the ActiveModel::Validations mixin to make the checking automatic upon initialization:

require 'active_model'

class ValidatedObject
  include ActiveModel::Validations

  def initialize(&block)
    block.call(self)
    check_validations!
  end

  def check_validations!
    fail ArgumentError, errors.messages.inspect if invalid?
  end
end

For my importing and parsing purposes, I created a small custom TypeValidator:

 # Ensure an object is a certain class. This is an example of a custom
 # validator. It's here as a nested class for easy access by subclasses.
 #
 # @example
 #   class Dog < ValidatedObject
 #     attr_accessor :weight
 #     validates :weight, type: Float
 #   end
 class TypeValidator < ActiveModel::EachValidator
   def validate_each(record, attribute, value)
     return if value.class == options[:with]

     message = options[:message] || "is not of class #{options[:with]}"
     record.errors.add attribute, message
   end
 end

Here’s the finished ValidatedObject, and an example subclass: information about a CSV feed. This set up is working great; the only change I can see making soon is changing ValidatedObject to be mixed in via include rather than subclassing.

Thanks to:

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