Rails Best Practices | the Law of Demeter

delegate設定

app/models/item.rb

  belongs_to :user

  delegate :name, to: :user, prefix: true, allow_nil: true

userが存在しないitem

Item.first.user.tapp
  Item Load (0.3ms)  SELECT "items".* FROM "items" LIMIT 1
  User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 0 LIMIT 1
nil
=> nil

delegateを介してuser.nameを呼ぶ

Item.first.user_name.tapp
  Item Load (0.4ms)  SELECT "items".* FROM "items" LIMIT 1
  User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 0 LIMIT 1
nil
=> nil

delegateを介さずにuser.nameを呼ぶ

Item.first.user.name.tapp
  Item Load (0.1ms)  SELECT "items".* FROM "items" LIMIT 1
  User Load (0.1ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 0 LIMIT 1
NoMethodError: undefined method `name' for nil:NilClass
	from (irb):149
	from /Users/shu/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/railties-3.2.12/lib/rails/commands/console.rb:47:in `start'
	from /Users/shu/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/railties-3.2.12/lib/rails/commands/console.rb:8:in `start'
	from /Users/shu/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/railties-3.2.12/lib/rails/commands.rb:41:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'