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Rails as it has never been before :)

clojure.rb

# If the last parameter in a method definition is prefixed with an ampersand, any associated block 
# is converted to a Proc object and that object is assigned to the parameter.
# It must be the last argument in the parameter list
def do_math_operation(amount, &block)
  block.call(amount) # OR yield(amount)
end

result = do_math_operation(5) {|amount| amount * 2}
 => 10

# A proc is a reusable object oriented block , a block is actually a proc that can't be saved
block.class => Proc

# The same above can be done using proc

def do_math_operation(amount, proc)
  proc.call(amount) # yield won't work!
end

multiply_by_2 = Proc.new do |n|
  n * 2
end

result = do_math_operation(5, multiply_by_2)
 => 10

# The same above can be done using lambda
def do_math_operation(amount, lambda)
  lambda.call(amount) # yield won't work!
end

multiply_by_2 = lambda{ |n| n * 2 }
result = do_math_operation(5, multiply_by_2)
 => 10

# Differences between Lambda and Procs
# 1- Lambda checks for number of parameters , throw an exception if less or more parameters were passed
# ex:
lambda = lambda{ |str1, str2| "#{str1}, #{str2}"}
lambda.call('str1', 'str2')
=> str1, str2
lambda.call('str2','str2','str3')
ArgumentError: wrong number of arguments (3 for 2)

proc = Proc.new{ |str1, str2| "#{str1}, #{str2}" }
proc.call("str1")
=> str1, 

# 2- Lambda in a method will return the value to the method and the method continues normally, while Proc stops method execution

def proc_return
  Proc.new{ |n| puts n }
  puts 'see me if you can'
end

def lambda_return
  lambda{ |n| puts n }
  puts 'hi, i am here'
end

proc_return
=> Proc
lambda_return
=> hi, i am here

# Note proc can't have a return in it , while lambda can

# The same can be done using method objects
def do_math_operation(amount, method)
  block.call(amount) # yield won't work!
end

def multiply_by_2(n)
  n * 2
end

result = do_math_operation(5, method(:multiply_by_2))
 => 10

# Method objects will act like lambda , only lambda is anonymous

concern_module.rb

# app/models/concerns/users/csv_conversion.rb

class User
  module CsvConversion
    extend ActiveSupport::Concern

    included do
      def to_csv(options = {})
        CSV.generate(options) do |csv|
          csv << %w[id username email]
          all.each do |user|
            csv << [user.id, user.username, user.email]
          end
        end
      end
    end
  end
end

# app/models/user.rb
include CsvConversion

# Use modules for more complex and widely used logic

# Using modules/plugins
# in lib/plugins/sponsorable.rb
module Sponsorable
  extend ActiveSupport::Concern
  module ClassMethods
    def acts_as_sponsorable(configuration = {})
      # Do your logic and define the needed associations ex: product has many sponsors,
      # A sponsor may sponsor many products , yes a polymorphic association
    end

    # If you forgot this line all instances of ActiveRecord::Base will have these methods !
    include InstanceMethods
  end

  module InstanceMethods
   def sponsors_count
   end
  end
end

ActiveRecord::Base.send(:include, Sponsorable)

# config/initializers/extension.rb
require 'sponsorable'

# app/models/product.rb
acts_as_sponsorable({ max_count: 5, sponsors_type: 'exclusive' })

# Use modules/plugins for shared complex logic and use concerns for model related simple logic

decorator.rb

# Decorators let you layer on functionality to existing operations, i.e serve a similar purpose to callbacks.
# For cases where including callback logic in the model would give the model too many responsibilities, a Decorator is useful.

# Imagine we have this logic on stackoverflow:
# A user can post a question on a stackoverflow, after posting an email notification is sent to
# all users have interest in the question topic, a post is shared on the user profile with a link to the question he posted

# BAD
# app/models/question.rb
after_create :handle_after_create_logic
private
  def handle_after_create_logic
    # queue email to be sent
    # create post
  end
# The question model should not be responsible for all this logic

# Good
class SOFQuestionNotifier
  def initialize(question)
    @question = question
  end

  def save
    if @question.save
      post_to_wall
      queue_emails
    end
  end 

private

  def post_to_wall
  end
  def queue_emails
  end
end

# app/controllers/questions_controller
def create
  @question = SOFQuestionNotifier.new(Question.new(params[:question]))
  if @question.save
    # Success
  else
    # Error
  end
end 

demeter_model.rb

# Law Of Demeter: Every unit should have limited knowledge about other units, simply don't talk to strangers!
# 2 dots are fair enough!

# BAD
class Invoice < ActiveRecord::Base
 belongs_to :user
end

# In model/view
invoice.user.name

# Good
class Invoice < ActiveRecord::Base
 belongs_to :user
 delegate :name, to: :user, prefix: true
end

invoice.user_name

meta_tag.rb

# It is common in any rails application to have some meta tags in your HTML as title, description, keywords , fb meta tags,...

# BAD
# In app/views/layouts/application.html.erb
<meta name="description" content="<%= @description%>" />
<meta property="og:image" content="<%= @image_url %>" />
# In each controller
@description = 
@image_url =

# Better
# in app/views/layouts/application.html.erb
<meta name="description" content="<%= yield(:description) %>" />
<meta property="og:image" content="<%= yield(:image_url) %>" />
# In each view
content_for(:title, @product.title)
content_for(:title, @product.description)

# Better

<%=
  og_tags(standard_og_stuff(@product), {
    :type => 'website',
    :other_tag => 'something'
  })
%>

# app/helpers/application_helper.rb
def og_tags(*tags)
  content = tags.reduce({}) do |result, set|
    result.merge! set
  end

  raw(content.map do |key, value|
    tag :meta, content: value, property: "og:#{key}"
  end.join("\n"))
end

# Then a helper method that pulls standard attrs (name, desc, image, ...) from a piece of content:

def standard_og_stuff(content)
  { title: content.name,
    description: content.description
  }
end

model_without_database.rb

class ContactForm
  include ActiveModel::Validations
  include ActiveModel::Conversions
  attr_accessor :name, :email, :body
  validates_presence_of :name, :email, :body

  def initialize(attributes = {})
   attributes.each do |name, value|
     send("#{name}=",value)
   end
  end

  def persisted?
    false
  end

end

nested_layout.rb

# Imagine that your application requires a layout that differs slightly from your regular application layout to support one particular controller
# The solution is nested layout!

#inside your different layout controller add:
layout 'custom'

# app/views/layouts/custom.html.erb
<% content_for :stylesheets do %>
  <%= stylesheet_link_tag "custom" %>
<% end %>
<% content_for :content do %>
  <div id="right_menu">Right menu items here</div>
  <%= yield %>
<% end %>
<%= render "layouts/application" %>

# app/views/layouts/application.html.erb
<html>
<head>
  <title>My Application</title>
  <%= stylesheet_link_tag "layout" %>
  <style><%= yield :stylesheets %></style>
</head>
<body>
  <div id="top_menu">Top menu items here</div>
  <div id="menu">Menu items here</div>
  <div id="content"><%= content_for?(:content) ? yield(:content) : yield %></div>
</body>
</html>

performant_model.rb

# Avoid N+1 Queries

# Example Application:
# User has many followers 

# BAD
user.followers.collect{ |f| f.user.name }
# Queries generated
# select followers where user_id = 1
# select user where id=2
# select user where id=3
# ..................

# Good
user.followers.includes(:user){ |f| f.user.name }
# Queries generated
# select followers where user_id = 1
# select user where id in(2,3,..)


# Counter Caching
# we want to get the number of retweets of a tweet

# BAD
tweet.retweets.length # Load the tweets into array and then call .length on that array

# Better
tweet.retweets.size OR tweet.retweets.count # make a count query to the db

# What if the above query is made in a loop? many queries right?
# Best using counter caching
# Add column retweets_count to the tweet model
# app/models/tweet.rb
belongs_to :original_tweet, class_name: 'Tweet', foreign_key: 'tweet_id', counter_cache: :retweets_count
has_many :retweets, class_name: 'Tweet', foreign_key: 'tweet_id'
tweet.retweets.size # No query , only look at the cache as the 'tweet' object is already fetched

policy.rb

# Sometimes complex read operations might deserve their own objects, Policy Object is the right solution.
# Policy Objects are similar to Service Objects, but it is conventional to use services for write operations
# and policies for read.
# They are also similar to Query Objects, but Query Objects focus on executing SQL to return a result set,
# whereas Policy Objects operate on domain models already loaded into memory.
# app/policies/twitter_policy.rb

class TwitterPolicy < Struct.new(:auth)

  def first_name
    auth['info']['name'].split(' ').first
  end

  def last_name
   .....
  end

  ....
end


# app/policies/facebook_policy.rb
class FacebookPolicy < Struct.new(:auth)
  def first_name
    auth['info']['first_name']
  end

  def last_name
   .....
  end

  ....
end

# app/models/user.rb
def self.from_oauth(auth)  
  policy = "#{auth['provider']}_policy".classify.constantize.new(auth)
  create_user_from_policy(policy)
end

# Check https://github.com/elabs/pundit for maximum usage of policies and OO design
# Pundit: Minimal authorization through OO design and pure Ruby classes

presenter.rb

# Presenters are created by Controllers/Views and know the current state of the view and the model
# Can be used to filter away tangled logic in the view
# Also can be used to skin the controller

# An example of a presenter used to remove logic from view(this is a small example , views can get it even more bloated with logic)
# app/views/categories/index.html
<div class="images">
  <%if @category.image_url%>
    <%= link_to image_tag("/images/#{@category.id}.png") + "some extra text", category_path(@category), class: 'class-name', remote: true %>
  <%else%>
    <%= link_to image_tag("/images/default.png") + "some extra text", category_path(@category), class: 'class-name', remote: true %>
  <%end%>
<div>

# app/presenters/category_presenter.rb
class CategoryPresenter < BasePresenter
  presents :category

  def featured_image
    image_path = category.image_url ? category.id : "default"  
    link_to image_tag("/images/#{image_path}.png") + "some extra text", category_path(category), :class => 'class-name',     :remote => true
  end

  def top_brands
    @top_brands ||= category.top_brands
  end

  def top_keywords
    @top_keywords ||= category.top_keywords
  end
end

class BasePresenter
  def initialize(object, template)
    @object = object
    @template = template
  end

  def self.presents(name)
   define_method(name) do
     @object
   end
  end

  def method_missing?(*args, &block)
    @template.send(*args, &block)
  end
end

# app/views/category/index.html
<% present @category do |category_presenter|%>
<div class="images">
  <%= category_presenter.featured_image%>
<div>
<% end %>

# app/helpers/application_helper.rb
def present(object, klass = nil)
 klass ||= "#{object.class}Presenter".constantize
 presenter = object.new(object, self)
 yield(presenter)
end
  
# To access presenters from controller
# app/controllers/application_controller.rb
private
 def present(object, klass = nil)
   klass ||= "#{object.class}Presenter".constantize
   klass.new(object, view_context)
 end

scope.rb

# Example Application:
# Product: name, price, brand, slug, description, delete_flag, image_url
# Category: name,slug
# Product belongs to Category

# Use scopes instead of chained queries scattered all over your application

# Get all available apple products with price > 1000 and not older than 1 week
# BAD
products = Product.where(brand: "apple").where("price > ?", 1000).where(delete_flag: false).where("created_at > ?", 1.week.ago)

# GOOD
scope :price_bigger_than, -> { |min_price| where("price > ?", min_price) } # Yes you can pass parameter to scope
scope :for_brand, -> { |brand| where(brand: brand) }
scope :un_deleted, where(delete_flag: false)
scope :newly_products, -> { |start_date = 1.week.ago| where("created_at > ?", start_date) } # Yes the scope parameter can have a default
scope :unavailable, where(delete_flag: true)

# Default Scope
# The default_scope is also applied while creating/building a record. It is not applied while updating a record.
# So, in the following example, new records will have delete_flag set to false upon creation 
default_scope :available, where(delete_flag: false)

Product.create().delete_flag  # Will be false


products = Product.for_brand("apple").price_bigger_than(1000).newly_products

# What if we want to override the default scope?!
# Get all apple unavailable products

products = Product.for_brand("apple") # Wont work, will get only the available
products = Product.for_brand("apple").unavailable # Wont work too, will get only the available

products = Product.unscoped.for_brand("apple").unavailable # works perfectly (Y)

# Now we want to get products updated today
# One can make
scope :most_updated, where("updated_at > ?", Date.today) # Watch Out!!!
# A scope is defined at initialization, so as you start up your server. Date.today gets executed once and a scope is created so a call to most_updated will return all the products that are updated on the day you start the server!

# This must be me done using Lambda OR without scopes
scope :most_updated, -> { where("updated_at > ?", Date.today) }

# Note: -> AND lambda are equivalent

# You can define scope dynamically
# Example for an api with api_keys
Status = { normal: 0, whitelisted: 1, blacklisted: 2}
Status.each_pair do |k,v|
  scope "status_#{k}", where(status: v)  # ApiKey.status_whitelisted,...

  define_method "status_#{k}?" do # api_key.status_whitelisted?,...
    self.status == v
  end
end

service.rb

# Use service object when:
# 1- The action is complex.
# 2- The action reaches across multiple models.
# 3- The action interacts with an external service (e.g. posting to social networks)
# 4- The action is not a core concern of the underlying model (e.g. sweeping up outdated data after a certain time period).
# 5- There are multiple ways of performing the action (e.g. authenticating with an access token or password). 

class Authentication
  def initialize(params, omniauth = nil)
    @params = params
    @omniauth = omniauth
  end

  def user
    @user = @omniauth ? user_from_omniauth : user_with_password
  end

  def authenticated?
    user.present?
  end

private

  def user_from_omniauth
    # Authenticate with omniauth
  end

  def user_with_password
    # Authenticate with password
  end
end

validation.rb

# Not good

class User < ActiveRecord::Base
  validates :appropriate_content

  def appropriate_content
    unless # some validation check on name
      self.errors.add(:name, 'is inappropriate')
    end
  end
end

# Better
require 'appropriate_validator'
class User < ActiveRecord::Base
  validate :name, appropriate: true
end

# /lib/appropriate_validator.rb
class AppropriateValidator < ActiveRecord::EachValidator
  def validate_each(record, attr, value)
    unless # some validation check on value
      record.errors.add(:attr, "is inappropriate")
    end
  end
end