Example of value objects using Ruby's Data class

Last week, I wrote an article about how to create value objects in Ruby - the idiomatic way. This week, I will share some real examples of using the data object to show some real examples.

Remove boilerplate constructor code

If you are defining classes and expose the initializer parameters as getters and you plan to make them immutable, then I think you just found the most common case for using the Data class:

Instead of this:

class Link
  attr_reader :url, :source

  def initialize(url:, source:)
    @url = url
    @source = source
  end
end

I write this:

class Link < Data.define(:url, :source)
end

You can of course also write the simple form, but I do recommend the the previous way with inheritance (will followup in another article about this):

Link = Data.define(:url, :source)

When calling an external API

When calling an external API that returns JSON, I like to implement a method that returns a Response object.

Here I define a Response object with two computed properties:

• parsed_body

• success

class Response < Data.define(:body, :status, :headers)
  HTTP_SUCCESS_STATUS_CODES = (200..299)

  def success?    = HTTP_SUCCESS_STATUS_CODES.include?(status)
  def parsed_body = JSON.parse(body, symbolize_names: true)
  def failed?     = !success?
end

Mind you that you cannot memoize using instance variables inside a Data class due to immutability. If you try something like this you will get FrozenError

class Response < Data.define(:body, :status, :headers)
  def parsed_body 
    @parsed_body ||= JSON.parse(body, symbolize_names: true)
  end
end

r = Response.new(body: "{}", status: 200, headers: {})
r.parsed_body
# can't modify frozen Response: #<data Response body="{}", status=200, headers={}> (FrozenError)

A more full example might look like this using httparty gem

require 'httparty' 
require 'json'

class Response < Data.define(:body, :status, :headers)
  HTTP_SUCCESS_STATUS_CODES = (200..299)

  def success?    = HTTP_SUCCESS_STATUS_CODES.include?(status)
  def parsed_body = JSON.parse(body, symbolize_names: true)
  def failed?     = !success?
end

def get(url, query: {})
  response = HTTParty.get(url, query)
  Response.new(body: response.body, status: response.code, headers: response.headers)
end

response = get(
  'https://bsky.social/xrpc/' \
  'com.atproto.identity.resolveHandle?handle=lucianghinda.com')
puts response.parsed_body[:did] # did:plc:1362asasdah213212
puts response.success? # true

From this example you can for example expand it to add a RateLimit object:

require 'httparty' 
require 'json'

class RateLimit < Data.define(:limit, :remaining, :reset)
end

class Response < Data.define(:body, :status, :headers, :rate_limit)
  HTTP_SUCCESS_STATUS_CODES = (200..299)

  def success?    = HTTP_SUCCESS_STATUS_CODES.include?(status)
  def parsed_body = JSON.parse(body, symbolize_names: true)
  def failed?     = !success?
end

def get(url, query: {})
  response = HTTParty.get(url, query)

  rate_limit = RateLimit.new(
    limit: response.headers['ratelimit-limit'].to_i,
    remaining: response.headers['ratelimit-remaining'].to_i,
    reset: response.headers['ratelimit-reset'].to_i
  )

  Response.new(
    body: response.body,
    status: response.code,
    headers: response.headers,
    rate_limit: rate_limit
  )
end

You can even add a constructor to RateLimit for example:

class RateLimit < Data.define(:limit, :remaining, :reset)
  def self.from_headers(headers)
    limit = headers['ratelimit-limit'].to_i
    remaining = headers['ratelimit-remaining'].to_i
    reset = headers['ratelimit-reset'].to_i

    new(limit: limit, remaining: remaining, reset: reset)
  end
end

Global or public list of objects from your domain

Here is an example I found in TheOdinProject:

# Source: https://github.com/TheOdinProject/theodinproject/app/models/flag.rb

class Flag < ApplicationRecord
  Reason = Data.define(:name, :description, :value)

  REASONS = [
    { name: :broken, description: 'Link does not work', value: 10 },
    { name: :insecure, description: 'Link is not secure or safe', value: 20 },
    { name: :spam, description: 'Spam or misleading', value: 30 },
    { name: :inappropriate, description: 'Inappropriate imagery or language', value: 40 },
    { name: :other, description: 'Other', value: 50 }
  ].map { |reason| Reason.new(**reason) }
end

Furthe on you will see they are using REASONS in a Rails enum:

# Source: https://github.com/TheOdinProject/theodinproject/app/models/flag.rb

class Flag < ApplicationRecord
   enum reason: REASONS.each_with_object({}) { |reason, hash| hash[reason.name] = reason.value }
end

And then in the view as a list of choices:

<% Flag::REASONS.each do |reason| %>
  <div class="relative flex items-center">
    <div class="absolute flex h-6 items-center">
      <%= form.radio_button(
        :reason, 
        reason.name, 
        data: { test_id: "flag-reason-#{reason.name}"}, 
        class: 'h-4 w-4 border-gray-300 dark:border-gray-500 dark:bg-gray-700/50' # ...
      %>
    </div>

    <div class="pl-7 text-sm leading-6">
      <%= form.label(
        :reason, 
        reason.description, 
        value: reason.name, 
        class: 'block text-sm font-medium text-gray-700 dark:text-gray-200 dark:text-gray-200' 
      %>
    </div>
  </div>
<% end %>

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