Presenter pattern made easy (in Rails)
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README.rdoc

Presenter gem

This gem is mainly targeted at Rails developers who:

  • need to execute a search based on request parameters

  • load records from a database for cached fragments

  • have a lot of view-related logic in models

It will help you to:

  • process request parameters (with simple typecasting)

  • cleanup your models and controller

  • postpone querying database until really needed (if needed at all)

  • … more …

Installation

As usual:

gem install presenter

To keep your code organized, create a directory “app/presenters” in your project and add this line to your environment.rb (or application.rb if you use Rails 3):

config.load_paths << Rails.root.join("app", "presenters").to_s

Now you can have your presenters in the app/presenters directory and they'll be loaded automatically.

Basic usage

Presenter:

# app/presenters/users_presenter.rb
class UsersPresenter < Presenter

  # following parameters will be extracted from the params hash and
  # properly typecasted. Nils, blanks and empty arrays will be discarded.
  # If a parameter's value is an array, all values in the array will
  # be typecasted
  key :name, String
  key :age, Integer

  # this creates method "users", which calls and memoizes method "find_user".
  presents :users

  private

  def find_users
    scope = User
    scope = scope.scoped(:conditions => { :name => name }) if first_name
    scope = scope.scoped(:conditions => { :age => age }) if age
    scope
  end
end

Controller:

# app/controllers/users_controller.rb
class UsersController < ApplicationController

  def index
    @users_presenter = UsersPresenter.new(params)
  end
end

View:

# app/views/users/index.html.haml
%ul
  - @users_presenter.users.each do |user|
    %li= user.name

Mixins

If some of the code in your models is used only inside views (formatting and such), you may extract this code to a module and mix it from the presenter:

# app/presenters/user_mixin.rb
module UserMixin
  def age_cca
    age < 20 ? "too young" : age >= 30 ? "30+" : age
  end
end

Then tell the presented to use the module:

# app/presenters/users_presenter.rb
...
presents :users, UserMixin
...

Now you can use the new method in your views:

# app/views/users/index.html.haml
%ul
  - @users_presenter.users.each do |user|
    %li
      = user.name
      = user.age_cca

Typecasting

These types are supported out of the box: Object, Boolean, Float, Integer, String, Time.

All of these can be nil, single value or an array of values.

It is also possible to add support for your custom types by implementing class method “typecast” for the given class. This would add support for price type:

# lib/price.rb
class Price

  attr :value

  def initialize(value)
    @value = value
  end

  def to_s
    if @value
      "$" + @value.to_s.reverse.scan(/\d{1,3}/).join(",").reverse
    else
      ""
    end
  end

  def self.typecast(value, options = {})
    new value.to_s.scan(/\d+/).join.to_i if value
  end
end

Note on Patches/Pull Requests

As usual:

  • Fork the project.

  • Make your feature addition or bug fix.

  • Add tests for it. This is important so I don't break it in a future version unintentionally.

  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)

  • Send me a pull request. Bonus points for topic branches.

Copyright

Copyright © 2010 Vladimir Bobes Tuzinsky. See LICENSE for details.