Rails - moving out calculations from my views?

Question

Currently I\'m performing some calculations in my views, which is a bad thing, of course:

<% categories.each do |c| %>
  ....
    <%= c.transactions.sum         


        

Answer 1

One mean to isolate view logic is to use presenters.

A presenter allows you to do something like that :

<% categories.each do |c| %>
  ....
    <% present c do |category| %>
    <%= category.transaction_sum %>
    <% end %>
  ....
<% end %>

You then have a presenter class in app/presenters/category_presenter.rb :

class CategoryPresenter < BasePresenter
  presents :category

  def transaction_sum
    category.transactions.sum("amount_cents")
  end
end

Of course, it is best used if you have many methods in that presenter (but once you begins to reduce view logic, it's quick to fill presenters).

The implementation used here rely on what is describe in this pro railscast. The basic idea is simply to have a #present helper that infers a class name based on object class, load and initialize the proper presenter class.

An other popular alternative is to use drapper, which use the concept of decorator, but a presenter is basically a decorator.