LetItBe

Copying over instance variables is bad, mmm-kay?

LetItBe helps you program to an interface, rather than an implementation in your Rails controllers.

The fact of the matter is that sharing state via instance variables in controllers promotes close coupling with views. LetItBe gives you a declarative manner of exposing an interface to the state that controllers contain and thereby decreasing coupling and improving your testability and overall design.

The Particulars

let creates a method with the given name, evaluates the provided block (or intuits a value when no block is passed) and memoizes the result. This method is then declared as a helper_method so that views may have access to it and is made unroutable as an action with hide_action.

Examples

In your controllers

When no block is given, let attempts to intuit which resource you want to acquire:

# Category.find(params[:category_id] || params[:id])
let(:category)

As the example shows, the symbol passed is used to guess the class name of the object you want an instance of. Almost every controller has one of these. In the RESTful controller paradigm, you might use this in #show, #edit, #update or #destroy.

In the slightly more complicated scenario, you need to find an instance of an object which doesn't map cleanly to Object#find:

let(:product){ category.products.find(params[:id]) }

In the RESTful controller paradigm, you'll again find yourself using this in #show, #edit, #update or #destroy.

When the code has become complex enough to surpass a single line (and is not appropriate to extract into a model method), use the do...end style of block:

let(:associated_products) do
  product.associated.tap do |associated_products|
    present(associated_products, :with => AssociatedProductPresenter)
  end
end

In your views

Use the product of those assignments like you would an instance variable or any other method you might normally have access to:

= render bread_crumbs_for(category)
%h3#product_title= product.title
= render product
%h3 Associated Products
%ul
  - associated_products.each do |associated_product|
  %li= link_to(associated_product.title,product_path(associated_product))

Beware

This is an exceptionally simple tool, which provides a solitary solution. It must be used in conjunction with solid design approaches ("Program to an interface, not an implementation.") and accepted best practices (e.g. Fat Model, Skinny Controller). In itself, it won't heal a bad design. It is meant only to be a tool to use in improving the overall design of a Ruby on Rails system and moreover to provide a standard implementation for an emerging best practice.

Documentation TODO

• walk-through explanation of the actual implementation (using an existing, popular OSS Rails app as an example for the refactor).