Rails : Presenters

18 May 2014

Recently I noticed that my view templates are becoming more complicated as there are so many complex conditions in it. As per the design patterns the view should not contain any logic, So I needed some way to make my templates clean. Thats how I came to know about Presenters.

Presenters is a abstraction layer between the template and the controller. It encapsulates the complex conditions in the view template to the presenter class, which will make our view more clean, readable and testable.

Consider this as my view template before using presenters,

  <!-- app/views/users/show.html.erb -->
  <dt>Website:</dt>
  <dd>
  <% if @user.website.present? %>
    <%= link_to @user.website, @user.website %>
  <% else %>
    <span class="none">None given</span>
  <% end %>
  </dd>
  <dt>Twitter:</dt>
  <dd>
  <% if @user.twitter_handle.present? %>
    <%= link_to @user.twitter_handle, "http://twitter.com/#{@user.twitter_handle}" %>
  <% else %>
    <span class="none">None given</span>
  <% end %>
  </dd>

This is a design smell. We are writing the conditionals in the template. How we can avoid this using presenters?

Using Presenters

Create a new folder named presenters in the app directory. Then create a UserPresenter class inside it.

# app/presenters/user_presenter.rb
class UserPresenter
  def initialize

  end
end

Now to use this, the easiest but ugly way is to let the controller know about the presenter.

# app/controllers/user_controller.rb
class UserController < ApplicationController
  def show
    user = User.find(params[:id])
    @user = UserPresenter.new(user, view_context)
  end
end

Passing the view_context to presenter help us to use the helper methods inside the presenter.

Now the updated UserPresenter will look like,

# app/presenters/user_presenter.rb
class UserPresenter
  def initialize user, template
    @user = user
    @template = template
  end
end

Now move the logic for the twitter and website into the presenter.

# app/presenters/user_presenter.rb
class UserPresenter
  def initialize user, template
    @user = user
    @template = template
  end

  def website
    if @user.website.present?
      @template.link_to @user.website, @user.website
    else
      @template.content_tag :span, 'None Given', class:'none'
    end
  end

  def twitter
    if @user.twitter_handle.present?
      @template.link_to @user.twitter_handle, "http://twitter.com/#{@user.twitter_handle}"
    else
      @template.content_tag :span, 'None Given', class:'none'
    end
  end
end

So Now look into our view template

  <!-- app/views/users/show.html.erb -->
  <dt>Website:</dt>
  <dd><% @user.website %></dd>
  <dt>Twitter:</dt>
  <dd><% @user.twitter %></dd>

Does it look much better than what we had before? I know, Yes!

But using @template to call all the helper method is not fine, so we need to refactor it by adding a method missing.

# app/presenters/user_presenter.rb
class UserPresenter
  def initialize user, template
    @user = user
    @template = template
  end

  def website
    if @user.website.present?
      link_to @user.website, @user.website
    else
      content_tag :span, 'None Given', class:'none'
    end
  end

  def twitter
    if @user.twitter_handle.present?
      link_to @user.twitter_handle, "http://twitter.com/#{@user.twitter_handle}"
    else
      content_tag :span, 'None Given', class:'none'
    end
  end

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

Even better Ah!!. Try out and lemme know how you felt about presenters.

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