Ruby on Rails Decorator pattern Decorating a Model using SimpleDelegator
Example
Most Rails developers start by modifying their model information within the template itself:
<h1><%= "#{ @user.first_name } #{ @user.last_name }" %></h1>
<h3>joined: <%= @user.created_at.in_time_zone(current_user.timezone).strftime("%A, %d %b %Y %l:%M %p") %></h3>
For models with a lot of data, this can quickly become cumbersome and lead to copy-pasting logic from one template to another.
This example uses SimpleDelegator from the stdlib.
All requests to a SimpleDelegator object are passed to the parent object by default. You can override any method with presentation logic, or you can add new methods that are specific to this view.
SimpleDelegator provides two methods: __setobj__ to set what object is being delegated to, and __getobj__ to get that object.
class UserDecorator < SimpleDelegator
attr_reader :view
def initialize(user, view)
__setobj__ @user
@view = view
end
# new methods can call methods on the parent implicitly
def full_name
"#{ first_name } #{ last_name }"
end
# however, if you're overriding an existing method you need
# to use __getobj__
def created_at
Time.use_zone(view.current_user.timezone) do
__getobj__.created_at.strftime("%A, %d %b %Y %l:%M %p")
end
end
end
Some decorators rely on magic to wire-up this behavior, but you can make it more obvious where the presentation logic is coming from by initializing the object on the page.
<% user = UserDecorator.new(@user, self) %>
<h1><%= user.full_name %></h1>
<h3>joined: <%= user.created_at %></h3>
By passing a reference to the view object into the decorator, we can still access all of the rest of the view helpers while building the presentation logic without having to include it.
Now the view template is only concerned with inserting data into the page, and it is much more clear.
