Partials - when to use :object =>

When should I use ":object =>" or ":collection =>" to pass an object
reference to a partial vs reference an instance variable directly with
the partial?

I would value best practice/opinions/pros/cons on this.

I use collection to loop through a list. So, render :partial =>
'comment', :collection => @comments saves me a few lines of code over
putting a loop inside the view or partial. :object would be useful
when showing just a single record. So, maybe something like this
makes sense:

<h1><%= @post.title %></h1>
<%= @post.body %>
<%= render :partial => 'author', :object => %>
<%= render :partial => 'comment', :collection => @comments %>

This gives me a post, a partial for its author, and a partial for each