Fat Models Must Die - Speaker Deck

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fat models MUST DIE @arkham @steffoz

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let’s talk about Rails

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Model View Controller

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Skinny controller, fat model

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$ wc -l app/models/* | sort -rn | sed -n '2,4p' 625 app/models/activity.rb 407 app/models/task.rb 364 app/models/user.rb

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Owners Of our application business logic

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Knows too much or does too much God Object

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Is it wrong?

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Software Design

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Robert C. Martin Uncle Bob © Mike Clark

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Two values of software

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Software meets the current needs of the current user 2nd value

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wait,

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Software is easy to change 1st value

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Agile

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SOLID Principles

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Single Responsibility Principle

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SRP Every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class.

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A class should have one, and only one, reason to change SRP

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let’s talk about Rails (again)

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ActiveRecord::Base Persist and represent DB rows

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Control HTTP requests/ responses at high level ActionController::Base

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Where do we put business logic?

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ActiveSupport::Concern

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require 'active_support/concern' module NameGreeter extend ActiveSupport::Concern included do attr_accessor :name end def greet puts "Hi! I'm #{name}!" end module ClassMethods def build(name) self.new.tap do |instance| instance.name = name end end end end class User include NameGreeter end user = User.build("Mark") user.greet >> “Hi! I’m Mark!”

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class Document < ActiveRecord::Base include Taggable include Visible include Dropboxed include Omasake end

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app/models/concerns app/controllers/concerns Rails 4

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Logic separation

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Code reuse

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Rails Drama

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Models are still God Objects! Runtime

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class User include NameGreeter end User.ancestors.include? NameGreeter >> true

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Mixins are a form of Inheritance

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Gang of Four Favor object composition to class inheritance

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is-a Inheritance has-a Composition A cat is a pet A cat has a tail

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Easier to change

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No content

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OOP in RUBY

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OOP I did it again

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We need to calculate stats about football matches Scenario

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class Match < ActiveRecord::Base def first_half_win? fh_made > fh_taken end def second_half_win? sh_made > sh_taken end def first_half_loss?; ... ; end def second_half_loss?; ... ; end def first_half_draw?; ... ; end def second_half_draw?; ... ; end end

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Value Object

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Equality is dependent on their value rather than an identity Property

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class Score < Struct.new(:goals_made, :goals_taken) def win? goals_made > goals_taken end def draw? goals_made == goals_taken end def loss? goals_made < goals_taken end end

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class Match < ActiveRecord::Base def first_half_score Score.new(fh_made, fh_taken) end def second_half_score Score.new(sh_made, sh_taken) end end

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Struct #hash #eql?

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We are closing an e- commerce purchase using Order, Customer and Product objects Scenario

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Action is complex

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Action reaches across multiple models

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Action interacts with external services

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Command Object a.k.a. Services

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class ItemsController < ApplicationController def add_to_cart item = Item.find(params[:id]) AddToCart.new(current_user, item).execute! end end

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class AddToCart < Struct.new(:user, :item) def execute! cart.add_item!(item) end private def cart user.active_cart end end

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Encapsulate all the logic for a speciﬁc scenario

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Reuse logic in multiple contexts

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Filter events by full-text search, distance and category Scenario

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class EventsController < ApplicationController def index @events = Event.published if params[:lat] && params[:lng] && params[:distance] @events = @events.near(lat, lng, distance) end if params[:query].present? @events = @events.matching(params[:query]) end if params[:category_id].present? @events = @events.in_category(params[:category_id]) end end end

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Query Object

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Responsible for returning a result set based on business rules Purpose

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class EventsQuery < OpenStruct def scope(scope = Events.scoped) scope = scope.published if lat && lng && distance scope = scope.near(lat, lng, distance) end if query.present? scope = scope.matching(query) end if category_id.present? scope = scope.of_category(category_id) end scope end end

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class EventsController < ApplicationController def index @events = EventsQuery.new(params[:query]).scope end end

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DRY up your views

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class EventsQuery < OpenStruct include ActiveModel::Conversion extend ActiveModel::Naming def persisted? false end # ... end

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<%= form_for @query, as: :query, url: events_path, method: :get do |f| %> <%= f.text_field :query %> <% # ... %> <% end %>

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Send mail notiﬁcations after the creation of new comments Scenario

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class Comment < ActiveRecord::Base after_create :send_notification! private def send_notification! AppMailer.comment_submission(self).deliver end end

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Model hooks are for data integrity

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Do we always want notiﬁcations?

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Command Object!

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Decorator

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Proxy

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class CommentsController < ApplicationController def create @comment = Comment.new(params[:comment]) if @comment.save redirect_to blog_path, notice: "Comment was posted." else render "new" end end end

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class CommentsController < ApplicationController def create @comment = Notifier.new(Comment.new(params[:comment])) if @comment.save redirect_to blog_path, notice: "Comment was posted." else render "new" end end end

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class Notifier < Struct(:comment) def save comment.save && send_notification! end private def send_notification! AppMailer.comment_submission(comment).deliver true end end class Comment < ActiveRecord::Base end

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Exhibit a.k.a. Presenter

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The primary goal of exhibits is to insert a model object within a rendering context. Purpose

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It’s a decorator Unrecognized messages are passed through to the underlying object

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Augment models behaviour when it is needed

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= link_to @user.full_name, @user = raw sanitize(@user.bio) = image_tag(@user.gravatar_url)

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- user = UserPresenter.new(@user, self) = user.link_to = user.bio = user.gravatar

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require 'delegate' class UserPresenter < SimpleDelegator def initialize(user, context) @context = context super(user) # Set up delegation end def gravatar_url md5 = Digest::MD5.hexdigest(email.downcase) "http://gravatar.com/avatar/#{md5}.png" end def gravatar @context.image_tag(gravatar_url) end end

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Seamless integration

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OOP alternative to Rails helpers

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DCI Data, Context & Interaction

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Separate what the system is (data) from what the system does (behaviour). Purpose

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Use case

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Context Roles

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Transfer money between two accounts Scenario

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class Account < Struct.new(:owner, :balance) end account = Account.new("Bob", 500.0)

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bob_account = Account.new("Bob", 100.0) alice_account = Account.new("Alice", 50.0) context = MoneyTransferContext.new( bob_account, alice_account ) context.transfer(10.0) puts bob_account.amount # => 90.0 puts alice_account.amount # => 60.0

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class MoneyTransferContext < Struct.new(:source, :destination) module SourceRole def withdraw(amount) self.balance -= amount end end module DestinationRole def deposit(amount) self.amount += amount end end end

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#extend

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bob_account = Account.new("Bob", 100.0) alice_account = Account.new("Alice", 50.0) bob_account.extend SourceRole bob_account.withdraw(amount) # => 90.0 alice_account.withdraw(amount) # NoMethodError: undefined method...

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class MoneyTransferContext < Struct.new(:source, :destination) # ... def transfer(amount) source.extend SourceRole destination.extend DestinationRole source.withdraw(amount) destination.deposit(amount) end end

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Domain model Use case Role Data Context Interaction

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Give ﬁrst-class status to system behavior

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Improved spatial locality

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ENOUGH.

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Recap

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Rails is great!

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MVC/ActiveRecord cannot solve everything

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When in doubt, Composition is your ally

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Don’t be afraid of new objects

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Think of Rails as a dependency

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Whatever works best in your domain!

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Classes can be no longer than one hundred lines of code. 1st rule

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Methods can be no longer than ﬁve lines of code. 2nd rule

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Pass no more than four parameters into a method. Hash options are parameters. 3rd rule