Replace instance variable with local variable

A partial is a reusable view template, it allow you to modularize the components which make up a particular page into logical, cohesive pieces. When required data is not passed into a partial, it is often difficult to reuse or change later.

By passing the required data in as locals you create self-documenting requirements for each view partial. It also helps to see where else it is rendered and what locals are required.

Bad Smell

class PostsController < ApplicationController
  def show
    @post = Post.find(params[:id])
  end
end

<%= render :partial => "sidebar" %>

In this example, the partial sidebar can use the instance variable @post, but we can't know what's the instance variable @post without checking the controller codes. Let's use the local variable instead.

Refactor

<%= render :partial => "sidebar", :locals => { :post => @post } %>

or

<%= render "sidebar", :post => @post %>

Now we can use the local variable post in partial, it is much simpler.