How to downgrade a rails gem

This is for someone who wants to downgrade a rails gem. For example let’s take devise gem.

  1. In Gemlock file add the downgraded version you are interested in.
gem 'devise', '~> 2.5'

2. run bundle update devise

Important notes:

  1. Do not runbundle update as it updates all the gems.
  2. Never make changes directly to Gemfile.lock
  3. Make sure to restart spring if running in background.

Ruby singleton_class

Let’s learn about singleton_class with an example. I have a Car which knows to drive.

class Car
  def drive
    puts "I am driving"
  end
end

car = Car.new
car.drive # I am driving

Cool so what! Now at later point of time I want to add honk to my car only when it is stuck in traffic. I don’t want it to be added for every Car object.

There might be more than one way of doing this in ruby but in the interest of this post i will implement this with singleton_class.  Ruby singleton_class inserts a new anonymous class into the inheritance hierarchy as a container and adds the new method to it.

class Car
  def drive
    puts "I am driving"
  end
end

module Honking
  def honk
    puts "Get away on my way"
  end
end

car = Car.new

traffic = true # more logic here

if traffic
  car.singleton_class.include(Honking)  
end

car.honk

p car.singleton_methods # [:honk]

race_car = Car.new
race_car.honk # undefined method `honk'

singleton_class creates a new singleton class if car does not have one. From then what ever changes you make on that is going to be affected to that particular class. Thats the reason race_car doesn’t know anything about honk.

I also used singleton_classto add additional validations to the object in a specific context.

You can check singleton methods on a class using singleton_methods.

Happy coding!

Ruby mixins explained

Hi Devs,

In this post i will try to explain how to use ruby mixins for DRYing up your code. Let’s say I have Car which can drive and honk. It can find a car with vin number or return count.

class Car
  def self.find(vin)
    puts "return car with VIN number #{vin}"
  end

  def self.count
    puts "return number of cars"
  end
  def honk
    puts "I like to yell at people"
  end

  def drive
    puts "will try to avoid speed tickets"
  end
end

# A Car can drive, honk

car = Car.new
car.drive
car.honk

# Find car by its VIN number
Car.find("123")
# Get total number of cars
Car.count

Now we get a requirement to add Trucks to our inventory which do most of the things aCar does. Let’s start DRYing the code.

First I will extract instance methods using a module and use include to make them available. In ruby we use include to add instance level methods from a module.

module VehicleBehavior
  def honk
    puts "I like to yell at people"
  end

  def drive
    puts "will try to avoid speed tickets"
  end
end

class Car
  include VehicleBehavior

  def self.find(vin)
    puts "return car with VIN number #{vin}"
  end

  def self.count
    puts "return number of cars"
  end
end

Making progress. Now i will extract the class level methods into a module and use theextend keyword to make them available. In ruby we use extend to add class level methods from a module.

module VehicleBehavior
  def honk
    puts "I like to yell at people"
  end

  def drive
    puts "will try to avoid speed tickets"
  end
end

module VehicleData
  def self.find(vin)
    puts "return car with VIN number #{vin}"
  end

  def self.count
    puts "return number of cars"
  end
end

class Car
  include VehicleBehavior
  extend VehicleData
end

Good. But is there a way to combine both modules into one ? yes.

Every time a class includes module – Ruby will trigger the self.included method on that module. It will also pass class as a parameter. In our case the Car class will be the klass argument.

module Vehicle
  def self.included(klass)
    klass.extend(ClassMethods)
  end

  module ClassMethods
    def find(vin)
      puts "return car with VIN number #{vin}"
    end

    def count
      puts "return number of cars"
    end
  end

  def honk
    puts "I like to yell at people"
  end

  def drive
    puts "will try to avoid speed tickets"
  end
end

class Car
  include Vehicle
end

Looking good. But is there is a cleaner way to do this YesActiveSupport::Concern

require 'active_support/concern'

module Vehicle
  extend ActiveSupport::Concern

  def honk
    puts "I like to yell at people"
  end

  def drive
    puts "will try to avoid speed tickets"
  end

  class_methods do
    def find(vin)
      puts "return car with VIN number #{vin}"
    end

    def count
      puts "return number of cars"
    end
  end
end

class Car
  include Vehicle
end

car = Car.new

Car.find("blah")
car.drive
car.honk

ActiveSupport::Concern makes the syntax better and also has the advantage of  gracefully handling module dependencies.

Now our Truck class is as simple as this

class Truck
  include Vehicle
end

Pretty cool right? Less code! Less bugs!! YaY.

Rails date validation pit falls

When ever you want to validate a date field against a time frame don’t forget to add context to it. For example I have the following class

Movie – name, release_date and collections

class Movie
  validates :release_date, presence: true
  validate :release_date_future?

  private

  def release_date_future?
    if release_date < Date.today
      errors.add(:release_date, "can't be in the past")
    end
  end
end

This Class looks absolutely legit. Let’s say I have a Movie object called Avengers endgame.

Movie.new(name: ‘Avengers endgame’, release_date: ’04-26-2019′).save

So far so good but when i want to update the `collections` at later point of time(after the movie got released) i can never do that because the Movie obj goes invalid (as it once again checks the release_date with current_date).

so the right thing to do is to add context to the date validation. Like we are interested in only while creation.

validate :release_date_future?, on: :create

Hope this helps.

Rails: Kick a logged in user(devise) out of his session

Have you ever thought of kicking a specific logged in user out of their session (for some weird reason).

If you are using devise for authentication then the short answer is you can’t.

I tried many ways to hack around to fool devise into thinking that the user session is expired but no luck. The user session cannot be accessed by other users (like from the rails console or database level).

The closest i thought I came is to trick the Timeoutable  hook. But it depends on last_request_at which is taken from the user session.  I tried messing with db fields like

current_sign_in_at, last_sign_in_at but realized that devise does not look at these fields once the user logs in.

Over all, the conclusion is that we can’t mess around with Devise which does its job well.

Note: You can still clear all the sessions for all the users using the following ways, depending on where you stored the session :

  1. Cookie Store (default) :

    Fleet::Application.config.session_store :cookie_store, key: _change_me_session.

    When you change the key the old sessions expire.

  2. Redis as session store: redis-cli flushall or delete sessions using a wildcard if we know part of the key $redis.del $redis.keys('session*').
  3. Database: If the sessions are stored in the database rake db:sessions:clear.

 

Rails: can not be used with :count => 1. key ‘one’ is missing.

If you ever encountered this error probably you are trying to add/access a I18n key for a `enum` attribute. For example if my model has a `enum` value like below.

class Shipment < ApplicationRecord
   enum delivery_option: {"arrival_notice":"arrival_notice", "arrival_schedule": "arrival_schedule"}
end

If we want to internationalize the `values` as

Call/notify before delivery and Delivery appointment required

. In your en.yml you need to specify as below.

en:
  activerecord:
    attributes:
      shipment:
        delivery_option:
          one: 'Delivery Option'
          arrival_notice: "Call/notify before delivery"
          arrival_schedule: "Delivery appointment required"

I18n expects the name of-the field as the first key/value.If you forget that then you see this error. Happy coding!