Rails strategy for reusing common behavior in models

A while ago I had an interesting problem to solve. Three of my modules reused the same functionality, they all had to parse address fields from a given lat and long.

The following are the list of attributes each module needed to accommodate this,

lat
long
sublocality
city
state
country
pincode

They also had to implement the following methods,

extract_address_components!
area
full_address

As a developer your primary concern for maintainability should always be DRY.

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system

The DRY principle was formulated by Andy Hunt and Dave Thomas in their book The Pragmatic Programmer. Paraphrasing from Wikipedia; When the DRY principle is applied successfully, a modification of any single element of a system does not require a change in other logically unrelated elements. Additionally, elements that are logically related all change predictably and uniformly, and are thus kept in sync.

Rails 4 introduced a way of sharing code as modules between modules. They’re called concerns and they sit right in your models folder. You can then include these modules in any model you want! It would look something like this,

module AddressExtractor
  extend ActiveSupport::Concern

  def extract_address_components!
  end

  def area
  end

  def full_address
  end
end

And then include it in any of model file like this,

class Map < ApplicationRecord
  include Authentication
end

This works great…..for awhile.

Concerns lose their usefulness when you need to share common behavior across applications. A quick Google search would reveal that the ‘popular’ advice to do share code across applications is,

Whip up a microservice and expose the common behavior as an API

I don’t know about you, but I don’t have servers and domains available all the time! To share functionality across applications, just create a Gem! You can host it on Github and add it to your Rails applications like this,

gem 'address_extractor', git: 'https://github.com/skcript/address_extractor.git'

In your Gem, create a module that looks like,

module AddressExtractor
  REQUIRED_ATTRIBUTES = [
    'lat',
    'long',
    'sub',
    'city',
    'state',
    'country',
    'pincode',
    'address'
  ]

  def address_extractor
    extend ClassMethods
    include InstanceMethods

    self.new.has_all_required_attributes?
  end

  module ClassMethods
  end

  module InstanceMethods
    def has_all_required_attributes?
      missing = REQUIRED_ATTRIBUTES - attributes.keys

      if missing.any?
        raise "#{self.class} does not have the required attributes: #{missing.to_sentence}"
      end
    end

    def full_address
    end

    def area
    end

    def extract_address_components
    end
  end
end

ActiveRecord::Base.extend AddressExtractor

There’s three things to watch out here,

1. The REQUIRED_ATTRIBUTES constant

This is a simple array of all attributes that your Gem requires to function.

1. The address_extractor function

This function ‘extends’ a class called ClassMethods and ‘includes’ a class called InstanceMethods. This allows you to write all the methods that your model class needs inside ClassMethods and all the methods that your model object needs inside InstanceMethods. The has_all_required_attributes? simply checks if all the attributes are present in your model. Nifty right?

1. Extending ActiveRecord

In order for AddressExtractor to easily bind with your models, extend it to ActiveRecord itself.

Pro Tip: If you don’t want to create a Gem or use Concerns, you can include the above module in your lib directory. Then create an initializer that requires AddressExtractor (require 'address_extractor').

Finally, all you have to do is add address_extractor to any of your models.

class Map < ApplicationRecord
  address_extractor
end